I wouldn't really worry about getting this sorted as it is the first major obstacle of many AFC's new to the game. It is defo something that everyone has to get sorted in order to feel good, get attention from girls, fit in with other people etc etc
1. A lot of people tend to dress to fit into their social surroundings. So I am guessing you are not surrounded by the most stylish guys. Common amongst new AFC's.
2. Some men have women in their lives, female friends or sisters that help them sort out their style. You may also not have this.
3. Many people may dress towards the style of their role models like musicians, actors etc etc
4. The most common is that people go to shops that are known as fashionable. Good labels make high quality stylish clothes. Going to a good shop means that even if you manage to come out of there with the worst outfit, you are probably still going to look stylish.
My advice for your fashion would be to gradually improve it with a goal in mind. Start with a new pair of jeans, then some shoes, then a leather jacket, hairstyle and so on. These are just some examples, the point is that you will get comfortable with your style slowly. If you just get a whole new outfit, you may feel insecure as you may feel 'its not me'.
In terms of teeth, firstly as many people have crooked teeth as people who have straight teeth. Use that to keep your morale up whilst you set up a plan to get them straightened. I understand it is a lot of money. So what. Dont buy that next xbox game or weekend treat. Get a job and aim to get them straightened. I do think that anyone with crooked teeth should get them straightened. It shows you care about your smile and that you understand what it means to have a good smile. It's not really vanity but fitting into society whilst feeling confident.
I am getting my teeth sorted, like you they are crooked, I recently bumped into some people that I knew growing up (same age). They also have crooked teeth still, difference is I am doing something about my (took a while but at least I am

) but they are not. It made me feel great really. See what I am saying now?
Weight...just again, look around at other people when you go out next time. Trust me. See how it works now? Instead of looking at the people who are in the best position...great teeth, muscular, confident...and feeling like shit.
Swith your frame and instead look at the people who are worse than you or will be worse than you when you go about getting your goals. It will make you feel better.
Good luck man, I recon 90% of AFC's are in your shoes...old shoes! In need of a fashion upgrade.
PS. On a more literal note, check out the website AskMen.com, it will help you with fashion and so on.