Well, take the screen shot by using the print screen button.
Then, you open a program like Paint and press CTRL-V
Your picture should then show up on the screen. Then you save the picture to whatever folder/ name you want.
Then go to a site such as photobucket.com
You might want to create a free account, it makes things so much easier.
You then click on 'browse' in the uploader. You can now see the stuff on your computer through photobucket.
Find your photo using this dialog. Once you do that, then click upload.
Depending what speed internet you have and how big the photo is, this could take a few seconds to a few minutes.