well, good thing you want to learn, so let us help you get started at least.
first off, when you go to a store, do not believe those people who will sell you gpu's and say it's 2gb blah blah. it's like selling a vehicle that can sit 12 people, sure it's nice. but, does it have the muscle/power to push those? it's not a very good analogy, but the point is, having 2gb does not guarantee a good performance.
a good 1gb gpu will perform better than a low end gpu with 2gb.
Even if they give it 4gb (say a gt610, or a 520, or 710) it won't matter in games. They are just too slow.
a good starting point (at least for nvidia) is to get the middle'ish models (650Ti, 750Ti). Of course getting say a 760 (or x60 depending on the generation, like a 660) would be better but it depends on your budget.
nvidia's naming convention is easy enough to understand, the lower the model, the slower they are, the first digit is the series or generation. Example, a gt610 was replaced with a 710, a 640 by a 740. So if you see a 680, those are high end, and was replaced by a 780.. see the pattern here?
always do your research (and ask) before buying