To add/edit signature, go to the upper right hand corner, click on "Settings". Then on the left-hand side, scroll down a bit and click on "Edit Signature."
You may not be able to add a signature until you are a full member. You need 15 posts to become a full member.
Yes, you have to be patient. And remember that quality of post is more important than the quantity of them. Posting a lot in short time because you want to reach the required amount of posts may not be the meaning of the forum.