Not many signature campaign accept newbies nowadays. Anyway, once you have a higher rank at Bitcointalk check this topic (https://bitcointalk.org/index.php?topic=615953.0) and find a signature campaign that interest you - be it minimum post count or payment rate. If you check the individual topics, there will be instructions for you on how to 'register' for a signature campaign.

Most only require you to fill in a template and reply to the topic. However there is some that require you to register on their service/website first. Its greatly depend.