I'm a B cup. How can I get to a full C cup?

Impossible to tell without height, weight, photos and a physical exam. Best to make an appointment with a Board Certified Plastic Surgeon.

Thank you for the question. There is no accurate correlation between size of breast implant and cup size achieved.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "full C cup” etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
 The use of computer imaging technology may also be very helpful during this communication process.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this (and the attached link) helps.

Hello! Thank you for your question. In general, implant size does not
correlate with bra cup size. The cup size itself will vary from
manufacturer to manufacturer as well as who is doing the actual
measurements. Thus, cup size is never a reliable indicator for your
breast size. I typically encourage my patients not to communicate her
desires in cup size but more on the actual look and appearance. Given your desires, a breast augmentation, with an implant or fat grafting, would be an ideal procedure that would meet your expectations.  Breast augmentation is now the most popular procedure performed in plastic surgery today.  One tolerated very well by women with minimal downtime and high patient satisfaction.  Good
communication between you and your surgeon of your expectations is
warranted - choosing your surgeon wisely is the first step.  Discussion
of your wishes and having an honest and open dialog of your procedure is
mandatory (e.g, implant type, incision, placement, etc).   I have found
that photographs brought by the patient is helpful to get a
visualization of the appearance you wish for in terms of size, shape,
fullness, etc.  In addition, your surgeon's pre and postoperative
photographs should demonstrate a realistic goal for you of results.  Sizers are helpful during an in-office consultation.  Once this has
been accomplished, allow your surgeon to utilize his/her best medical
judgment during the procedure to finesse the best possible result for
you after preoperative biodimensional planning and fitting the right
implant for your breast width.  This is the most important determinant for the sizing of implants, which will provide the optimal result and most aesthetically-pleasing result for you.Consult with a board-certified plastic surgeon well-versed in breast procedures who will examine and discuss with you the various options and factors to consider and help you decide which one will be the right one for you.  Hope me that this helps!   Best wishes for a wonderful result!