I am going to try my best to condense everything into a small list.
I will try to lower the budget as much as possible, as you will phase out these items once you find your own style anyways, think of these items as cheap & stylish stepping stones.
Denim suggestions:- Levis 5 pocket jeans, the darker the better. SF recommends the classic 501 model. Don't worry about too much about the line that it comes from, no one will care after a couple washes. ($30+)
- APC New Standards. Your thighs don't look too small, so the New Cure or the other skinny jeans aren't for you. APC NS is the (mostly) undisputed king of starter denim ($155 shipped, Mauro will hook you up).
- Naked and Famous. If you are close to a store selling these, I recommend you trying these on, they are decently priced (~$70 or less on sale)
- RRL. RRL is great, the quality is excellent for their price, and the go on sale very often. You can even try your luck on eBay, some of the members have done it before.
The bottom line is to buy well fitting jeans without pre-ripped holes. You also cannot go wrong with a pair of well fitting dark denim (raw is good).
You can save up for a decent pair of denim and wear it all the time.
Shirt suggestions:
The usual suspects belong here, I will echo what others have been saying. J-Crew, H&M, Zara etc... all produce decent looking shirts. The trick to buy the shirts that fit you the best, and belong with the other pieces that you plan to wear them with. The basic colors to go for are Navy, Brown, Grey, Black etc... as they can be wore easily with everything without too much effort.
There is no real need for you go to out and spend a couple hundred dollars to revamp your entire wardrobe now, just stick around the forums a bit and learn a couple things, and soon enough, you will learn what's good for you, and what isn't.