Advice for Sunken Cheeks?

Looking at the photo, I do agree that your cheeks appear sunken in and improvement of this would enhance your appearance. Options are multifold and include cheek implants (i.e. submalar implants), facial fat grafting and injectable fillers. Examination and advice from a knowledgable surgeon who performs all of these procedures is a good start.

A wide variety of facial implants are currently available for the enhancement of the skeletal elements of facial aesthetics. The most commonly used facial implants are used to enhance the profile of the chin and the cheeks. Facial implants very widely not only in shape and style, but also in terms of the materials from which they are fabricated.
My preference is to use structural fat grafting, as much as possible, to enhance facial features. In many cases, the need for a solid implant can be eliminated by the careful and meticulous grafting of a patient's own fat. While the incidence of complications with facial implants is not high, certain problems may develop which require implant removal including infection and implant migration. Bone resorption (bone loss) has also been reported below solid facial implants. Neither of these problems are an issue with structural fat grafting.

The only setting in which I currently use a solid facial implant is for chin implant augmentation. Some patients with a 'weak' chin profile can be adequately improved by structural fat grafting alone. However, when the chin protrusion needs to be enhanced by a half centimeter or more, a chin implant is absolutely required. I prefer to use a soft, flexible, anatomic chin implant that conservatively enhances the anterior projection of the chin in profile. The chin implant is placed through an incision hidden underneath the chin, an area where many people already have a scar from a fall in childhood.
Moderate enhancement of chin projection in appropriate patients can dramatically enhance the profile, in a manner that is completely natural-appearing. It is remarkable how increasing chin prominence in some patients will reduce the apparent prominence of the nose, and restore an overall sense of harmony and balance to facial features.

For enhancement of the cheek bone area, I rely exclusively on structural fat grafting. Careful and meticulous fat grafting not only avoids some of the potential complications of solid cheek implants, but it also allows more precise 'customization' of the aesthetic improvement that can be obtained for each individual patient. It is also my opinion that structural fat grafting produces a more natural-appearing enhancement of the cheek area than what is generally achieved through the placement of solid implants over the cheekbone.

To perform fat grafting successfully and reproducibly a surgeon must employ careful preoperative planning, appropriate instrumentation and meticulous surgical technique.  It is not something you can 'rush through' or spend just a few minutes on during a larger surgical procedure.  The unfortunate reality is that not everyone who performs fat grafting is willing to put in the level of education, training, investment in instrumentation and operative effort required to produce aesthetically ideal results.
One must also have an aesthetic vision for ideal and youthful-appearing facial fullness.   Just like traditional facelift surgery can be overdone or performed incorrectly to produce an unnaturally tight, pulled, 'windswept' look, fat grafting can be overdone to produce an excessively full and even bizarre postoperative appearance.  Too much fat grafted into any area (or any fat placed where it doesn't belong) looks unnatural.  It's analogous to breast augmentation results: if the surgeons selects an appropriate implant volume and positions the implants correctly, the patient gets a beautiful, natural-appearing breast enhancement.  If the surgeons stuffs a pair of 500cc implants behind the breasts of an average-sized patient, then that patient ends up with a cartoonish 'boob job'.  I think many of the unfavorable results in fat grafting are from the overzealous placement of excessive amounts of fat, which may have been the inevitable response to the recent paradigm shift in aesthetic facial surgery: away from the 'wind tunnel look', and towards the restoration of soft tissue volume.

Here is one patient’s experience with fat grafting posted to RealSelf:
Fat is great when used JUDICIOUSLY and CONSERVATIVELY, but too much of a good thing becomes a nightmare and deeply regrettable. Make sure your surgeon listens, draws up precise operative plans beforehand and makes plenty of time for you and your concerns. Do NOT take it lightly.

I couldn’t agree more with this patient’s response to her experience with fat grafting.  Just like breast implants, or liposuction, or browlifts, too much of a good thing is not at all a good thing.  And cosmetic surgery (and this includes the preparation for surgery) of any kind without attention to detail and a focus on natural-appearing results will leave patients feeling disappointed at best and feeling disfigured at worst.  Now regarding whether or not fat grafting is a surgically sound concept…
For transferred fat to truly qualify as a 'graft' the following must happen: living tissue must be transferred to a new location, and that tissue must gain a blood supply at the new location which provides oxygen and nutrients which allow it to persist indefinitely as living tissue.  We know that with appropriate instrumentation and technique this is achievable, so one of the opinions expressed in this thread that "most of the cells from fat injection are dead " is simply untrue (and structural fat grafting, to be clear, does not involve 'injection' of fat).  MRI studies have shown that with appropriate technique grafted fat persists long-term as living, vascularized tissue in the recipient site.

It is also well-established that adult human fatty tissue contains stem cells that have the capacity to repair damaged or injured tissues, and stem cells can be concentrated during the fat harvesting process.  This effect has applications in both cosmetic and reconstructive surgery.  Fat grafting is now being used, for example, as a means to stimulate the repair of chronic, non-healing wounds.  Several centers have reported on fat grafting immediately below non-healing chronic wounds resulting from radiation therapy for cancer, with rapid improvement and eventual healing of wounds for which no other wound treatment was successful.  The development of stem cell therapies involving the harvesting and processing of viable human fatty tissue is one of the hottest topics in both clinical and experimental medicine today.
When performed correctly fat grafting actually has the capacity to heal, revitalize and rejuvenate the local tissues at the recipient site.  I have treated a number of patients with facial fat atrophy following overly aggressive 'non-invasive' rejuvenation treatments including Thermage, Fraxel and IPL.  Most of these patients report not only an aesthetically  pleasing and permanent improvement in facial soft tissue volume, but also an improvement in the quality and vitality of their facial skin.

I perform extensive facial fat grafting during most of my facial rejuvenation procedures, and it is the very first thing I do - before making any incisions to lift the brows, eyelids, face or neck.  Fat grafting allows me to obtain results that are simply not possible with conventional, subtractive surgical techniques alone.  Patients frequently return for grafting of additional areas after their initial experience with strucutral fat grafting.  To characterize it as the pointless and potentially harmful placement of non-viable tissue is an opinion only, and it is an opinion with which quite a number of plastic surgeons who successfully rejuvenate faces (and breasts and bodies) with fat grafting would vehemently disagree, myself included.
Solid fat/fascia and fat dermis grafts are another means by which facial soft tissue volume may be significantly and permanently enhanced.  As with fat grafting, survival of the grafted tissue is variable and some of the graft material is reabsorbed.  As with fat grafting, potential complications such as infection and cyst formation are possible.  And as with fat grafting, appropriate preoperative planning and surgical technique are required in order to obtain ideal and lasting results.  In my opinion structural fat grafting is more versatile, as fat can be easily and rapidly added to any tissue plane (level); and it has the added benefit of the ‘stem cell effect’ which is difficult to quantify but unquestionably present in many cases.

Both structural fat grafting and fascia-fat (or dermis-fat) grafting can be competently performed (or not) by Board-certified plastic surgeons.  Neither should be trademarked or considered proprietary.
As with any surgical procedure take time researching your plastic surgeon. Schedule several consultation, view many photos from many patients, each from multiple perspectives (start by looking at photos on this website) and speak to former patients of any plastic surgeon you are considering.

As we age and move from adolescents into adulthood we lose more "baby fat." Such as it is in your case. You are developing a more chiseled appearance. This usually goes in line with a more slender or athletic body physique.
First I would recommend not doing anything at this time. You are young and it is likely that your weight will change over the next several years. If you continue to be dissatisfied with your appearance in a few years then I would recommend micro fat grafting.
Micro fat grafting involves removing fat from the body via liposuction with very small cannulas (I use a 16guage or 1.4mm cannula). This fat is then process and the processed fat is injected with a 1.2mm cannula. There is a very good "take rate" with this process.