Press the button beside F12, it has "Prt Scr/SysRq" on it.
Or if you happen to use a laptop, you'll need to first press and hold some function key (Fn?usually beside the Ctrl key) follow by the Prt Scr key. The screenshots will be stored under your trickster folder.
I don't think you can upload the pictures directly onto this forum...or can you? >.>
You'll have to first upload it on a image hosting website like this
and get the url of the pic, then use the "Insert Image" function in forum.