How do you level up ?

You make giveaways.

http://www.steamgifts.com/about/faq

"What are levels, and how can I increase my level?
Levels represent a user's contributions to the community. When a user creates a giveaway that's received by the winner, their level increases in value, allowing them access to additional giveaways as a reward. Levels range from 0 - 10, with level 0 being the default after registration, and level 10 being the highest level attainable. If you mouseover your level in the navigation bar, or the level on profiles, you'll see a tooltip with the level rounded to 2 decimal places, giving you a better idea of small increments in level, and when you're about to level up.
Levels increase almost exponentially, meaning each level requires double the contributions of the previous level. For example, level 4 would require double the contributions of level 3, and level 3 would require double the contributions of level 2."

Have you checked out the FAQ? There's a section called "What are levels, and how can I increase my level?" that might help you out... :P

I think its oviously how to

By reading the FAQ