There are a lot of nasty viruses floating around. I'm not trying to scare anyone, but just giving a word of precaution. All right, first, what's a signature file you're asking. If you've noticed, sometimes there is a message that appears at the bottom of an e-mail that's always the same. Sometimes it has contact information such as name, e-mail, company phone number, etc.
Some people use it to promote their web site.
Or other people just put a nice text based design. Recently, I've been using my signature file to promote my Web site. Whenever I send an e-mail, it reminds the receiver that Collectibility exists. Here's how you set up your signature file. Open up a text editor - if you're using Windows, the Notepad in the Accessories folder will work fine.