Press the 'Print Screen' or 'PrntScr' button on your keyboard, usually top-right by the F12 key. This takes a screenshot, which you can then 'paste' into any image editor software (MS Paint, for example) for cropping and editing. You can then save it and upload it somewhere online (ImageShack, PhotoBucket, your own website, etc...).
You then put your image on the forum by putting the address between [img] tags, such as:
[img]http://mywebsite.com/myimage.jpg[/img]