Coming from someone who isn't actually 'fashionable' but felt uncomfortable in his own skin for a long time - find things that fit/are comfortable that you like before anything else. Something that fits you and that you like will always look better than some expensive brand that doesn't fill either of those two criteria.

If you're comfortable in what you wear, it'll reflect in your posture and confidence. Case in point: the t-shirts I was wearing for a long time felt tight-fitting and like they were dipped in starch. That's 'cause they were made in material that didn't fit me.

So I got shirts from fangamer.net that were American Apparel - those things feel fantastic. I didn't realize it at the time but I also have a super broad chest and the shirts compensated for that. And they feel like wearing a blanket over your body. And the bonus was that I really liked their designs. All of those things together reflected themselves in how I carried myself.

So you might have to try a couple of different types of the same type of clothing before you find something that fits while being comfortable.

Also, I would say that cleanliness is 60% of 'dressing well'. Always make sure you're washing your shirts and stuff.

And if you get a suit, for the love of all that is holy get that shit tailored.

I'm also talking out of my ass, though, so Insomnia's advice is probably better.