Locate the Print Screen button on your keyboard. It's usually around the top right, and will be labelled something like "Prt Sc". It saves a copy of the screen/window into the computer's memory.
Open your favourite graphics program, ready to use. Windows Paint will do just fine, and I'll refer to that here but you can use whatever you like.
Have the exact window(s) you want to get a picture of showing on your screen.
Go to Paint.
Paste from the memory by pressing <Ctrl><V>.
Save the image by pressing <Ctrl><S>. Save the file in a place you'll remember, and give it a meaningful name. Use a suitable format - jpg is usually a good option if you just want someone else to see what you're seeing, and creates a smaller file than some of the other options.
You're done! That file can now be used elsewhere, including attached to an email or embedded in a web page.