Taking a screenshot and posting it is fairly simple, just follow these steps.
First, if you wanted to slack off, just download a simple program like Gadwin's
Screen Capture and Print Screen software . This allows you to take the screenshot and save it under a specified name and folder with just one click of some hotkeys. Only problem is if you don't know what your doing, it's pretty hard to set up.
For beginners, I'd recommend just manually doing it.
Pretty much all you have to do is:
1: Hit the Print Screen button which should be right next to your Scroll Lock button. This copies your screen. Make sure you don't copy anything after this until you have your screenshot saved/
2: Open up paint or photoshop or whatever image editing program, and paste the screenshot onto it.
3: Crop it or do what you like with it.
4: Save the screenshot (make sure to put a name and the file extention as .jpg or .png, otherwise the screenshot's filesize might be too big)
5: Go to a image hosting website like imageshack.us, and click "browse" and find the screenshot from where you saved it under (make sure its in an extention like .jpg or .png because that lowers the filesize. Many image hosting sites have filesize restrictions and some formats might be too big)
6: Click host, and after a while a new page should pop up with a bunch of URL's. Find the one specified for forum links (it should say beside the various links on what its used for), and simply copy and paste it onto the forums.
If anything's confusing, just post again and I'll try to help you out.