How do I get more guns? [duplicate]

This is the new "Arms Deal" update which went live yesterday (14/08/2013). Valve released a really good FAQ on it, which can be found here.

The bits of interest for your particular question, "how can I get new guns?":

• Timed weapon drops, just by playing on official and community servers in any game mode.
• Opening dropped weapon cases with a weapon case key.
• Trade weapons with other players through Steam's Trading interface.
• Purchase them from other players through the Steam Marketplace.

Regarding the first point, these are obtained for play time over the course of a week. As your play time increases, the drop rate will slow down. This is reset when the week rolls over. Not sure the start day or time of a week though.

I'm not 100% sure where you will get the weapon case keys. The below quote suggests that you buy these:

This update will also actively support the CS:GO competitive community with the introduction of the eSports Weapon Case. A portion of the proceeds from sales of eSports Case Keys will go towards prize purses at competitive events.

Hope that helps.