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.
In winning signature campaign you must first a jr member above. Only few campaign who accepts newbie to their campaign. Next is reached their goal. Every campaign has it own rules that you mus follow. And when you did what they want you to do then it will surely lead you to a successful signature campaign that will give you bitcoin.