Do you see a "print screen" button on your keyboard? Often you have to press it while holding down the Shift key. Once you've done that, the image is saved to your clipboard. Now open a new document in a word processing program and "paste" (usually, Ctrl +V).
The button often says "Prt Sc" right on top of "SysRq" on the same key. Mine is on the top row, way over to the right, near the delete and insert keys. If you're on a PC, your should be too. If you're on a Mac, I've not nuttin' for ya.
If you want to take a screen cap of just the internet browser window, hold down Alt + Print Screen.
The Print Screen button alone will take a screen capture of your whole monitor. Alt + Print Screen will just capture the active window.
There's also a really good prank you can pull where you do a screen capture of someone's desktop with all the icons, and then you set the image as their desktop background. Then you hide all their real icons, and they spend a reeeeeallly long time clicking on things trying to make them work. Not that I've ever done that. *ahem!*