You can join a pooled mining effort, but with a CPU you can currently expect to generate appox. 0.02 - 0.06 BTC per day. That is if you keep your CPU running 24/7 at full power.
Heh, you think that's bad? I have
two $400 GPUs and all I get get out of them is 0.00003 BTC per day. Things have changed, my friend.