A "screenshot" is a captured picture of your computer screen.
Open RoR, make sure whatever you want to show us is visible, and press the PRINT SCREEN button on your keyboard (sometimes prt scr).
Then you either:
Go to Documents\My Games\Rigs of Rods\screenshots and upload the picture from there (if you want to show us just the game window)
or
Paste the image (ctrl+v) into your graphics editor of choice (mspaint for example, if you type that into start menu it should show up as Paint) and save that somewhere and upload in your reply. (if you want to show us your whole screen)