I am a 21 year old looking for advise on how to approach receding hairline? (photo)

when you begin to lose hair at a younger age, the probability is greater that you will experience significant hair loss over time.  This rule is not set in stone, however, this is certainly possible.  I do not recommend a hair transplant at this point, however, you certainly would benefit from one.  What I do recommend is products to help you maintain your hair.  Propecia is one.  This product does have side affects that may not show up for years including depression.  Then there is minoxidil.  Finally, consider WNT act, a new product that promotes hair growth.  This product is plant based, stimulates the WNT pathway, and increases your density and diameter or hair mass.  You can buy WNT Act from amazon.com.  The Wnt pathway is the new way to approach and treat hair loss and no side effects are reported.  

Based on your photos and description I would not recommend Hair Transplantation at this time.Finasteride and/or Minoxidil would probably be of benefit now-I would suggest a thorough evaluation by a Dermatologist or experienced Hair Restoration surgeon to make a diagnosis and develope an appropriate treatment plan.

Most probably you have a androgenetic type hair
loss. I dont advice to use finasteride because of
possible side effects and being under
the necessity of using that chemical for almost life long to stop hair
loss.You would better planning to have an
exam at hair loss expert for hair restoration surgery when you fell
uncomfortable about hair styling or total loss of hair..Hair transplant can be an option for forehead reduction and correcting  asymmetry on your frontal hair line/forelock.I advice you have a FUE; you would get the desired hairline in just one sesssion of dense packing hair transplant. 

At your young age finasteride is an option. Other treatments such as PRP and laser may also help. Be sure to consult with a board certified plastic surgeon specializing in hair transplant to review all of your options.

Most men under 25 are too young for a transplant. It's a pretty simple rule. There are exceptions but this rule goes a long long way. Getting on treatment is important to save existing hair. A hair transplant could be in picture down the road with 800-1200 grafts depending on the rate of hair loss on treatment or not on treatment. Get connected with a knowledgable physician who can guide you. I'd advise seeing a few physicians and getting different opinions. 

Your biggest enemy is the progressive thinning leading to a balding pattern that is not evident at this point in time. The degree of thinning that you are experiencing would suggest that you should seea doctor who most likely will get Bulk Measurements of your hair to determine the degree of the thinning that you are presently having. If the diagnosis is early genetic hair loss (the most common cause of hair loss in young men), then the best treatment would be the drug finasteride which is highly effective in men of your age and may not only stop the hair loss but possibly reverse it. If you are developing a more advanced balding pattern the best test to get will determine the Bulk Measurements of your hair and this will show, over a one-year time frame, how effective the treatment will be. Once you know this information, then you and your doctor need to develop a MASTER PLAN which will define what you need