I think you're a very pretty young lady and I agree with the others don't alter your cheeks. Looking young is an advantage. Think about this: if you end up looking old (say you go from looking 12 to looking 32) when you're 40 you will look 50! It's not worth it. Wait a few years. Believe me you'll thank us when you're 40 and you barely look 25!
However if you're dead set on getting it done (and again I encourage you to stay your pretty self) I would discourage from getting the implants, they will make you look artificial and bloated. My suggestions, here they are but run them by a few doctors, to make sure. Better yet tell them what look you desire. If you don't know how you want to look, you'll wake up unsure if you like what you see.
Your best bet would be minimal lower cheek lipo. 1 surgery. I imagine this would create the definition you seek and still leave you looking natural. Maybe some along the jawline as well. If after you heal you need more definition then small size cheekbone implants in a 2nd surgery.
Leave your lips alone you look natural now. Lip asymetry is typical. Look at vintage pix of Farrah Fawcett. Her bottom lip was larger than her top. If you're dead set on trying some plastic surgery I would go with a nose tip job to refine the roundness a little but don't touch the nostrils. Have you had your bridge done ever? It looks very straight to be natural.
Whatever you decide I wish you good luck and post some photos if you have anything done.
Pictures of me. Opinions appreciated. 
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