So many guys have no idea how to dress, so kudos to you for taking the initiative! Being shy can be tough, but you can get yourself out of your shell a bit simply by dressing confidently.
You are 100% correct about everything fashionable not working for everyone. I don't think you should go down the "trendy" route per say. Go for classic cuts that flatter your body type. Some basics you should start with: a pair of dark-wash straight-leg jeans, a pair of black dress pants, tan dress pants, dark colored button-up shirts, a nice fitted sweater (NO, not the tight turtleneck ones--opt for a looser version), and some everyday shoes like brown dress boots.
When you walk into a store, look around before you do anything else. If you look at clothes and think: these aren't for me, then move on. American eagle is a great store for basics that are a little more casual. They have nice khaki pants and jeans, and a few fun buttons up shirts. Express Men's is another store I recommend. There, you will find dressier stuff, like black and tan dress pants, and some dressier tops.
Don't be afraid to ask sales people for help--that's their job and you will probably save them the boredom of standing around with nothing to do. First, look around the entire store. If you find something you might like, try it on, or ask a sales person if they think it would work well for you, and what to wear with it. They will be glad to help, I promise.
And the most important rule is to try everything on! Make sure that it fits properly before you buy it (ask a trusted friend or a salesperson).
For basic everyday wear, I always love guys in nice fitting jeans (not too short or too tight and something with a dark wash). Steer clear of really light wash Levis. They are not flattering on anyone. For dressier pants, a nice khaki pant is always good. For these, it is worth it to spend a little money because you will get more wear out of them. So they should fit really well.
You can actually find nice ones at the gap or men's express. For casual-wear, t shirts are always good. I'm a fan of the ones that have a bit of a vintage feel. Just make sure that the shirt is fitted. Most guys buy shirts way too big and the end result is that they look sloppy. It shouldn't be ridiculously skin-tight, but it should lay against the skin a bit. For dressier shirts, I LOVE express for men. I promise they aren't paying me to tell you this, but that store is great for guys.
They have really nice button up shirts for $25 and they fit really well, because they have a bit of stretch in them. They have so many different colors. Do yourself a favor and stay away from pink and purple. Not many guys can pull it off and it's too tricky. Go for a blue shirt to match your eyes. Probably any darker color would go well with your hair...dark green, burgundy, etc. So those are all the basics. Get yourself a good pair of jeans, a nice pair of khakis, a few fitted t-shirts, and a couple button down shirts and you should be good to go.