RRB
Your profile shows you are using Windows 8. I’m using Windows 10 which is different to what you will actually see when you try the below advice, but you should be able to get your snip.
I have also shown how to resize the ‘snip’ so it’s a reasonable size to show in a forum.
The resizing is more important when full size screenshots/snips are wanted to be included, as they often can ruin the layout/flow of a forum and the admin/mods are likely to remove the screenshot/snip which defeats the object of including one so readers can see what you see so they can try and help you out.

Click the Start Button, then simply start typing the word snipping.
When you see the Snipping Tool below the search box, press the enter key (not the same in W10 as in my video below, but will show in W8).
Now open the Word document with the Navigation Pane showing.
Click the Snipping Tool icon in the Taskbar to show it again, then click New in the Snipping Tool box.
The screen will dim, and you can draw around the area you want to ‘snip’.
Save the snip to a location of your choice and give it a suitable name.
Now close the Snipping Tool window, and minimize or close word.
Now locate the snip and then right click.
Click the Edit entry to open the snip in Microsoft Paint.
Click the Resize tab and change the horizontal setting to 70%. The vertical will automatically change to suit. Click OK.
Now save the change.
Close MS Paint and the close or minimize the saved location window.
Now open your Browser and navigate to the forum thread. Be logged in.
In the reply box, click Go Advanced.
Type any wording that you might want to appear above the snip, then click the Attachment (paper clip) icon.
In the window that opens, click the first Browse tab, then navigate to the saved snip.
Double click the snip in question, then click the Upload tab.
When you see your attachment showing, close the attachment window.
Scroll down to the preview post tab, and click to view your post to see how it looks before going live.
Submit when you are happy with it.
Click the image below to see a short video of the above.
Again, I’m using W10 and W8/8.1 will be a little different but the above should get you there.
There are other methods to resize images but Windows always has a version of Paint included, so that's why I have opted to use that one as it is a simple one.