Assuming it's win/mac: I highly recommend using http://puush.me/ It installs a piece of software on your computer and you simply press Ctrl Shift 4, it will create a mouse PLUS LOGO and you can highlight the area you want to take a screen shot of or screen capture.
It will then send that selection to there database and send you back a direct link to the image, then you just save the jpg on your computer.
It also automatically puts that link to your (Ctrl+V) spot for faster use. Puush also keeps a record of all the screenshots you've taken under "My Account" with a 200MB limit
Hope that helps, good luck
And obviously the normal way for windows is to press Prt Scr Button on your keyboard, it will take a screenshot of the entire screen, then you open up paint, Paste the results of the screenshot, and then you can edit whatever you want and save, but in your situation, i don't recommend this.