Once you become a participant in a signature campaign, your job is to advertise their service/product by posting here in the forum while wearing their signature and avatar (not yet applicable for your rank). You must also obey all the rules of the campaign manager e.g. making 25 posts per week, in a constructive English Language etc. for you to get qualified for the payment. (rules vary depending on the manager, it can be found in the beginning of the thread including the deadline, spreadsheet and other important details)

For your second question, it's a yes! As long as your post has a sense and not within the restricted sections of this forum such as Archival, Politics, Beginners and Help, Local Boards etc. (You can find it also in the rules). But remember, you should avoid spamming and outbursting posts in just one day.

For the third, I'm sorry but you can't join because this means that you will unwear your signature during the campaign and it may affect your payment. But you can still join social media campaigns because it doesn't required any signature or avatars.

For the last, since you exert effort and time in advertising their service/product, therefore, you deserve a reward in return which is either in a form of btc, altcoin or USD.