As I'm using Windows 7, I'm unable to check on whether TTT's suggestion will work. However I'm a bit concerned about the image format that might be used by a screenshot saved by that method. I've got a feeling that it might be something other tham .jpg (such as .png), which might not be accepted for upload to the web.
If you can't get that method to work, here's an alternative which (to the best of my knowledge) is univeral across all versions of Windows:
1. Press the Print Screen key (ON ITS OWN).
2. Open Paint. (You should find it listed, via the Start button, under Accessories).
3. Click Paste.
4. Go to 'Save As' (NOT just 'Save') and select Jpeg as the file format.
5. Save the image file to anywhere convenient on your computer (such as to the desktop).
6. Then upload to
https://imgbb.com/ as previously suggested.