I'm not asking how you meet women, but rather for advice. Mostly beacuse in my almost 21 years of living I've never really had a girlfriend, and not having a large social circle either, how does one go about with simply meeting someone?

Do you go to college/university? Classes are a good place to start. Or through friends, at parties or something. If not that, then perhaps at a bar of some sort?

You just... meet someone.

Going looking for it is usually a bad idea. You'll meet a lot of women if you have a broad social circle, it's as simple as that. To broaden your social circle, join a sports club, or take up some classes, or change jobs, or start going out in your neighbourhood. The more people you know, the greater the chance that one or more of them will be nice women.

Usually going outside.