If you want an actual signature you'd have to do it as an image. That would then have the disadvantage that people could copy that image and use it to apply the signature to all sorts of other documents.
The way to do it so that the text looks a bit like a signature is to use a cursive font to style the text.
I think it's a very bad idea to place you signature on a website, the potential for forgery, fraud or anything else would be seriously increased, they could take your signature and place it on any number of legal documents signing you up for stuff you have no knowledge of (identity fraud), personally I don't know anyone who would freely give out something as sensitive as their signature on the web. My advice, don't even think about it