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 ??
Read the campaign's topic, there you will find all the procedures and rules.
Bear in mind if you go around spamming just to increase your post count, moderators will ban your account, so don't post if you don't have nothing of value to add.