Dude you even have a girlfriend! How can it be possible that you don't know how to socialize? Anyway, here's how things are: When you talk to someone, try to think of where they come from and what they want. Serve their needs on a silver plate when talking to them and they'll like you. Suggest staying in touch in some method. Do this with everyone you meet. You'll become very social, but be aware that this might consume you, as you will generally spend more time thinking for others and doing what others want instead of thinking for yourself and doing what you want.

I hope this helps.