I think this is the main question that arise as soon as you get your gamespot account; and the answer is really simple: you start using the site -browse reviews/previews, watch videos, post in forums, write reviews etc-.

Now, for all the other questions you might have (and that I had), here's a list of links I gathered:

Leveling up guide (with max daily percentages)

Emblem guide

What do I get from leveling up? 

Why my level is reset to 0 or 1?

All the guides

If you have anything else interesting that you think that I should have here please let me know.