Just look at some of the other signatures in the posts to get an idea, basically they give a little information on your system, OS, CPU, RAM, avast version and what security application you are using, etc.
Use a text editor like notepad jot it down all on one line and then copy and paste it into your Profile.
Click the Profile button at the top of each page, from the new page, select the Forum Profile Information, now scroll down to the Signature: box and paste your information into that.