to take a screenshot, at least on a laptop, press the function (fn) button and the insert/prt sc(print screen) button at the same time.
open microsoft paint and paste it and save it.
once you have a screenshot saved on your computer, come to the forum to make a message. there are 6 sections of things to do in messages, and each is sorted by a "|". the first set is "B (bold) I (italic) u (underline). the third set has three as well; click the first. it will prompt you to browse for the photo you want to upload.
upload the photo, and paste the link in your message. be sure to use spoiler tags if the photo is large enough to distort the normal forum viewing area