You earn credits by winning in official servers. Buying a booster will allow you to earn more credits, as well as giving you the ability to get creds in user created servers.
There's also surveys you can do here: http://forum.toribash.com/tori_survey.php
Careful what you pick though, it's run by a 3rd party so we're not responsible if you have to download something.
<Erf> SkulFuk: gf just made a toilet sniffing joke at me
<Erf> i think
<Erf> i think i hate you