there are two ways to do this, the easy way is to use the snipping tool, just click it then clik the area and your done, but for the people that dont have this here is a guide to help

(1) Taking The Screenshot

Press the Print Scrn key on your keyboard. It can be found above the Insert key and to the right of the F12 key. Anything on the screen is now in the computer's memory, or the 'Clipboard.'

Hint: A key combination of Alt + Print Scrn will take a screenshot of only the current window, making cropping of your new picture much easier!

Warning: you CAN'T take another screenshot, or copy anything (using cut and paste) at this point! The screenshot has been saved into your computer's temporary memory, and it will be deleted if you copy anything, take another screenshot or turn your computer off!

(2) The Picture

Open up Paint or another graphics program like Photoshop or PaintShop Pro, and paste it by pressing Ctrl+V.

(3) Cropping

Cropping is very important! Most of the time, you only want a specific part of the game screen, or maybe even the entire game screen. Crop out anything you don't need, like the side menus (unless you need it). You will need to use the selection tool in Paint to select the part you want, press Ctrl+C to copy it, Ctrl+N to make a new page, and then Ctrl+V to paste it into a new document. Other programs have tools to crop and add effects that you can use.

In paint, you want to select the marquee tool, in the top left hand corner of the toolbax (in red):
Use the marquee or selection tool to crop!

Then just select whatever part of your picture you want, press Ctrl + C to copy it, press Ctrl + N to clear the excess part of the picture, and then paste what you copied with Ctrl + V.

(4) Saving Your New Screenshot

You can't forget this part! you need to save your new, editted screenshot

Since .bmp (a.k.a. bitmap) images are big and 'unfriendly' to the web, you should save it as a .gif or .png to make it smaller. In Paint, select the format you want to save in from the 'type' menu, and then type in a name with the same extension (.gif or .png).