There's a PRTSC (print screen) button on your keyboard. Pressing it creates a 'snapshot' of your screen. Open MSpaint, or any graphics program you have. Go to file > new and when the window opens go to edit > paste and the image will show. Go to file > save as and give it a name, save it as a jpeg. You'll need an image hosting site like photobucket or image shack, signing up is free. Once done follow the instructions to upload your image. Then come back to this thread and in the tool bar click on the green tree, a window will open.
In photobucket (one I use) click on the upload button, a window opens, inside click on 'browse for image' then go to where you saved the jpeg. Click on that image and it will upload to photobucket then click on 'view album'. When the image shows up mouse over the gear icon in the top right corner, a dialogue box opens, click on 'links', then click on 'direct link', it'll say copy. After you click on the green tree inside the dialogue box right click on the blank field and click paste. The url for the image will show up then click on 'insert' then ok and your screenshot will appear here.