Hi Welcome,  signature campaigns are a way for sites to advertise within the forum, these can be pay per post or a fixed rate.  This is a link to a thread which is updated showing all the current signature campaigns and their pay rate https://bitcointalk.org/index.php?topic=615953.0

The higher your rank in the forum opens you up to the higher rate of payment, yourself as a brand new / newbie will be restricted in what you can join, it will take some time but the more time you spend on here learning about the different topics and contributing to them the quicker your rank will increase and then you should be able to join a campaign.

Hope that helps