Make sure you know the difference between a sig and userpage.
A userpage is a supporter feature:
https://osu.ppy.sh/p/support You will be able to change it via your profile page when you are a supporter
Your signature can be changed here: ucp.php?i=profile&mode=signature
It will show up on the bottom of all your forum posts except in the Feature Request, Help, and Beatmap subforums.