That wasn't a dumb question...I didn't know either until someone tole me like last year.
Press Print Screen and Insert. Then open up MS Paint and paste (which is CTRL + V if you don't know). Then go to 'File' and 'Save As' then save it where ever as a JPEG file. THEN, when you sign up for Photobucket, ImageShack, DotPhoto etc. load it up and they should provide [IMG] (image) tags for you to copy and paste into your post when you decide to show us your hopefully nice desktop