My advice:
At the age of 21, I am going to guess you are around a junior in college. Men at that age typically do not dress well and make little attempt to do so. A button-up shirt is all that is needed for any sort of "dress to impress" occasion. People on this forum will likely point you to A.P.C., Nudie, Rag and Bone, etc. They are nice jeans, but they run rather narrow in my personal opinion (feel free to flame me everyone). For someone in your age group, I will recommend the following:
First: Return those shoes. Unless you have a fair amount of shoes or are well versed in the sartorial art, you don't want tan shoes.
Belt: Go to Express for men and pick up a brown belt:
http://www.express.com/rounded-buckle-leather-belt-33322-74/index.pro
It is made in the USA, durable, and looks good.
Jeans: Let us assume that an athletic build is anything with a drop of 8 or more. I also fit this criterion, although I am a bit taller (6' 4" and drop 10 if you're curious). I have tried on a great deal of jeans and in my humble opinion, the AG jeans "protege" model fits and looks the best. It is not really narrow, but it is not baggy either. A good straight fit jean. I would recommend a darker jeans over a lighter and one without distressing marks. The "Kent" wash is very good and manages to hide the goofy "AG" symbol on the back pocket due to the stitching being in a darker blue. The 2% elastane helps it not "suction" onto your legs for lack of a better description and I find they shrink less.
Found Here:
http://shop.nordstrom.com/s/ag-jeans-protege-straight-leg-jeans-kent-wash/3083500
Shoes: Everyone here will recommend Allen Edmonds shoes for new people, and with good reason. The "seconds" are extremely well priced and the blemishes are often indiscernible. FYI, a "second" is a rejected shoe with some minor defect so it is sold at a lower price. You can go either the "seconds" route or if you don"t mind dropping the extra cash, the firsts are not
that much more. If you want to add a little bit of style to your shoes without being flashy, I highly recommend the Allen Edmonds Berkshire. I have one myself. It is in the 4 last, which has (from my experience) the most room in the ball of the foot compared to the other AE lasts. It is a dark brown suede with brown chromexcel accents (kind of a reddish-dark brown). Women also seem to be a big fan of it. This shoe in a "second" will run you about $200. Alternatively, if you want something a little more mainstream and cheaper. The Wisconsin Allen Edmonds Shoe Bank is selling the seconds version of the Provo model for only $100. It is made of calfskin (I recommend the dark brown) and it is also made in the 4 last and sports a rubber sole.
Berkshire:
http://www.allenedmonds.com/aeonline/producti_SF4544_1_40000000001_-1
Provo:
http://www.allenedmonds.com/aeonline/producti_SF4214_1_40000000001_-1
You can take this advice or leave it. The above will not set you apart so much as to really stand out, but will give you a little more style than the guy next to you at the bar.