The real answer I have is find a girl who you want to be a good friend as well as someone you find attractive. Having someone around who can comfort you and make you feel better when you need it is worth far more than sex. Don't get me wrong, sex is nice too, but I was in a relationship I throughly enjoyed for 3 years and sex wasn't a major part of it.
Find girls who you have at least something in common with, given you're on here that might be gaming, or something linked to that like a good story or an appreciation for art. What's attractive to others is what we can see that's different from the people around them and we can relate to. As others have said, humour is important, finding someone who has a similar sens of humour to you is even more so, it's no use when one person likes British Wordplay comedy like Monty Python and the other likes watching people hurt themselves on funniest home videos. Humour is enormously attractive and my girlfriend and I bonded over one of her favourite jokes:
What's red and invisible?
No tomatoes
That led us to realise we both appreciate the surreal and don't quite live in reality, albeit in different ways.
The best piece of advice I can give for a long term relationship is let it take its own course and get to know anyone you're interested in as a person. Girls are people too and they've always responded amazingly well to me treating them as people first and girls second, particularly if you can get them out of a group. Getting to know someone properly is the real basis for building a relationship and lets you find out whether something's on false pretences or not.
My first relationship wasn't until 23, I did have a crush on a girl for a couple of years, which dissolved when I realised how deeply incompatible we were on certain levels. She would rather stand around talking to people she didn't know or like and be vaguely uncomfortable for a few hours than go downstairs and watch the Open air showing of Lord of the Rings. I went downstairs and met up with my first girlfriend, who was just a friend at the time, but it told me a great deal about my need to have someone I could share my entertainment with as being important to me. That girl also remained a friend after we realised we were drifting in different directions after 3 years and were simply good friends sharing a room. We're still good friends and I have the advantage of not losing that support even though I'm now in another long term relationship which sprang up unexpectedly a week after that one finished.