To add or change your signature, scroll down and click on the "My Settings" under the quick links on the left (under all of the forum categories). It should bring you to the Email Settings, but if you select Personal Information (under Email), the signature information is located directly under your First and Last name.
I hope that I was able to explain this clearly for you. If not please let me know!
I am not quite sure why you would not be able to start a new thread. Do you get an error message? If you could email a screen shot of what you see to QVCSocialTeam@qvc.com I would be happy to look into it for you.