Signatures.

I see people have signatures on here and I'd like one too, how do I edit my signature as when I click my profile etc all I see is edit profile picture, edit avatar, edit password and e-mail and other things to edit but I don't see an edit signature. Not sure if I'm posting this in the right place if I haven't posted this in the right place sorry. But if anyone could explain to me how I edit my signature it would be really useful. Also do I need a certain amount of posts before I have signature ?

Sorry if this is in the wrong place.