Loves to 7000, great advice.
I know what being broke is like. I get tons of great clothes at second hand stores, though. Try them on, at 5'5" 140, you're a bit smaller than me and my friends (6+, 180ish gym rats), but your clothes should compliment your frame. We typically go for a V look, try something that fits to your body -- not something that's loose all over.
My bit to add to this would be pay attention to how you're wearing your clothes. Do you have a jacket? If so I would undo two buttons on that shirt (no undershirt). I can't really tell what you're doing with your cuffs, but think about where you'll be. If you want to roll them up, make sure to do it properly. Italian style is where you pull the cuff up just past your elbow and roll the rest up till it's just below your elbow with a little bit of the cuff sticking out (Google some photos) and that's my personal favorite. I would leave at least the wrist button undone if you're keeping your sleeves out long.
Also, accessorize. Accessories can define your look. I usually go for a classy look with a watch, a wrist band I got in Italy and leather belt I got a steal on. You don't need to be rocking a Rolex to show off. Unless you're kicking it with I-Bankers and Lawyers, most people will be fascinated by a $100 Invicta you bought on sale at Newegg (I did that and I get at least 3 compliments every time I wear it out). But take 7000's advice about colours here, I wouldn't wear my gold and black watch with my green shirt, brown belt and brown shoes -- I'd wear my silver/blue (a great watch to wear all the time, $80 Citizen with blue back).
Above all else shoes. I am living in a hick town for the summer and I see so many people making so many style mistakes, but the biggest is shoes. Girls notice shoes. Go get yourself a decent pair of shoes (maybe you have some already?) and a belt to match them. Your belt should match your shoes. I see guys wear running shoes with dress shirts and it makes me cringe. Most guys don't pay attention to their shoes but most girls do.
Nice stuff comet. I agree very much with all of it (shoes bit especially, how I forgot to leave them out I do not know - very important as you say). But the point I'd just add to is on the small details of how you wear your clothes. I'm not going to argue with anything you've said because it's all good stuff, but just to supplement it, in paying attention to the little details of how you wear things you can 'peacock' far more subtly than having a big velvet hat on your head. For instance, I quite often wear my belt with the buckle pulled right over to one side. It's unnoticeable really, but it just adds a bit of a quirky element to it all which girls sometimes pick up on. An old Italian way is to have your watch on top of your shirt sleeve, which again, is largely unnoticeable but is just there and is just a bit different (certainly over in the uk where I've never seen anyone else do it). The examples of buttons you use is also a good one - if I'm wearing a tee shirt under my button up then I might have the top 2 or 3 buttons undone (as I'm sure most people would), but then I'll also leave the bottom one undone too. And maybe do the David-Beckham-esque one sided tuck as well.
These little things aren't going to make too much difference if you haven't followed the 'basics', but they are a great way to spice up an already fairly stylish outfit.