All the information that you seek is in the Meta section: https://bitcointalk.org/index.php?board=24.0

We have plenty of sticky threads at the top of the section outing the guidelines of the forum, and we have a search function you can use if you can't find what you are looking for.

As for signature campaigns. If you want to participate in one that pays decently, then you've got a long road ahead of you. As with any job you'll need to build up your experience, and demonstrate to the signature campaign managers that you would be a good investment as a advertiser. This usually means that your account is established for a few years, and makes quality, and well thought out posts.

My guess would be; these days most users would expect to be eligible for a Bitcoin paying signature campaign around the 3 years mark since there's so much competition, and the fact that you would need to establish yourself on the forum before hand.