You should sign a message from a well know (public) bitcoin address posted here at bitcointalk.org (and that post should not be edited). If you don't know how to sign a message (you probable don't own the private key, but there you will find a good and fully "how to" sign a bitcoin message:
Intro:Welcome to the world of bitcoin.
I have been a Newbie like you, looking for scrap, asking questions, working faucets. Faucets are a waste of time. Yes, it was a bit easier in the past. I could join a signature campaign with only 19 posts. Spammers have ruined this pretty much for Newbies though and if they continue I am afraid they will ruin it for all of us. This is not the topic though, sorry for the distraction. You dont believe me that I joined a campaign as Newbie? Well I can proof it to you and I can show you how. By signing a message.
The post quoted below was written by me. It was not edited and it contains a bitcoin address. If you click on its link you will see that it has no dotted line below the time and date. See
here for an edited post. I changed it today.
Here is a picture.
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