In signature campaigns, you pretty much put a message advertising something for the person who runs the campaign. Every campaign has its requirements. You can find a list of the campaigns available here:
https://bitcointalk.org/index.php?topic=615953.0Most of them require having a category of member of above, so you also need to post and participate in the forum in order to get that status.
Hope it helps.
Ok thank you for your answer but as a junior member, how to join the Signature Campaigns of WhyFuture and DIMCOIN for example ??
Do I have to visit their sites to register ?? And if I sign up, what will be the next step to succeed my Signature Campaigns ?? I really can not understand the procedure ??
First you need to be active here in forum so you can level up your account the more greater your rank the more they can pay you higher just go the other parts of this forum or go to newbie thread there that you can find some tips about joining in a signature campaign and the rules of the ranking system here but joining signature campaign does not pay you bitcoin.