You can check some active signature campaigns on service thread. You will be able to see the requirements needed before joining any campaign.

Common requirements
Having at least 5 merits in the last 120 days. But due to very limited users needed, do not see it surprising better accounts are taken that have more merits.

Most campaigns bare for full member and up

You are not qualified for all these, you need to be an active member, know more about bitcoin and also you can be active on some other boards of interest. If you have passion for bitcoin, knowing more about it, posting quality contents, obeying the rules of the forum, you will rank up.

You need to read these:
Unofficial list of (official) Bitcointalk.org rules, guidelines, FAQ
Merit & new rank requirements
Newbies - Read before posting