Screenshots can be taken anywhere but this page will use acrophobia as an example.

This method is the most common.

On the top row of your keyboard there is a key that say "print screen" .. go ahead press it!

Woo hoo you just took a screen shot!  ..  Oh nothing happened ?

Well you see your screen shot is stored but now you need to paste it somewhere!

If you have ms paint (everyone does) you can open that and choose  Edit then Paste,  your screenshot will appear!

Now ms paint isn't real practical for everyday use. For a small quick program I'd recommend ICEVIEW
 you can click on the word to take you too the site for a free download.

The reason I'm listing this particiliar imaging program as opposed to others is this. It has automatic image compression. In other words it will reduce your image file size and thus reduce load time on the web. This can be set in Ice View   under Tools/Options/File Formats.  You won't want to use a lot of compression for an acro screenshot because of clarity but it will still be 20 times smaller then what ms paint would leave you with!

Your path for image compression in ICEVIEW is  tools/options/fileformats/ jpeg compression level

Once the bar is set it automatically will save any image at that compression level.  You can play with this for Clarity.

Ok some examples!