Hrm, okay. Dunno much about what passes as cool in Lone Star territory. Generally speaking:

Mirror b_b's comments about jeans/tee. Go for a neck other than crew. V-neck has been safe for a good long while, henleys have also been pretty standard. Dish out for nice cotton. Stands up to washes and weathers better.

There is a Levi's style for everybody. No need to go for the spendy designer stuff. I'd say it's safe to go slim but not skinny (which isn't really the thing anymore), straight leg. Make sure they're not too loose. Don't think loose has been in style since Jncos.

Invest in a leather belt. Nothing fancy- brown is generally better than black, which is too formal for most occasions.

If you're gonna get button downs, go for oxford (because it generally stays in style, dependent on color/pattern, and it doesn't have to be ironed every goddamn wear), light basic colors. And at your size (which is roughly where I am), go with small and fitted if you can get it. Harder to find, but some small stuff you still end up getting these weird bunchy areas, which is awkward first semi-formal dance territory. Avoid that shit. So basically: make sure it fits, which should translate across several generations of style.

Shoes are the hardest because styles change so frequently. Where I live right now, brown leather is the way to go- desert boots have been the thing for a couple years, which means we're due for an overthrow. Red Wing's heritage stuff is starting to show up, as has some of White's stuff, although that's more mannered and thus will probably look silly in a couple years. Ugh, shoes are hard. Fuckin' Converse? Black? Can't go wrong, been the cool kid's shoe since like the seventies or whenever.

Dark socks.

Don't think anybody's said it yet: dish out for a good haircut. If you've got a couple good thread combinations but you're rocking a bowl cut, the whole thing is unsalvageable. Facial hair? Depends on your face. I still look like I'm twelve if I shave clean, so a little bit of shadow is a requisite. But if you've got a strong chin, flaunt it, I guess. Shit, you got a strong chin, it probably doesn't matter what you wear, man. So discard everything I've said and just have a good chin.