Pc
First press the Print Screen button on your keyboard (prnt scrn) IF on a laptop you may have to press and hold the fn key first.

Then go to the program paint and paste image into there. This can be done by hitting ctrl v or going to edit and paste.
Crop image and save.
To put your picture into the forum go to tinypic and upload the picture. Use the forum code and paste into your post

Mac
I run both both mac and windows so here is a very simple way to take screenshots on a mac.
First hold shift apple/command button and 4.
Then crosshairs will come up you then drag around the pictures to selelct the area you want to screenshot or hit spacebar and a camera will come up allowing you to take a picture of a entire window.
Then the picture will appear on your desktop you can upload to tinypic and use the forum img code or if you are reporting posts just browse and select the file.™®©