Could you give a few details about what you think the issues with your appearance are? It'd preserve anonymity and make it a bit easier to give advice!
Generally, though, I'd say that there are five things in your power when it comes to improving your looks:
Get your body to a size you're comfortable with. Hit the gym, go on a diet, or even try to gain weight if you feel you're too thin. Work out your ideal body and go for it in a healthy, safe way!
Wear clothes which flatter you and that you enjoy wearing. Dressing well can make an absolutely huge difference, both in the way it affects your confidence and how you can accentuate features you like. Check out people with a similar figure to you and work out what works and what doesn't.
Make sure your hair flatters your face and is what you love. I'd love to have really short hair, but I know it just won't suit me at all. Likewise, really long hair drags my face down; hair in the middle is perfect. Same as before, look at what other people with your face-shape do and see what looks great.
If you like makeup, wear it! It's cool if you don't want to, but if you do then learn how to apply it in the ways that you want.
The golden rule: always try to look the way you want to look. Don't try to please others with your body or clothes - confidence and contentment with your appearance will shine through.
For the hooking up, I'd say online is probably your best shout even though you've not had much luck with it so far. A lot of people here will be able to give you advice on how to make a good profile, so I'll just say that photos are absolutely key. Make sure they're well-lit, flattering (but not misleading) and clearly show your face as the main one and your body in at least one other one. If people can build up a good image of you in their heads, it really helps.