if you want a full screen screenshot, follow these steps:
1. hit the Prt Scr key
2. open your favorite/only graphics program
3. open a new file if neccessary (unless one is already open like when you start mspaint)
4. press Ctrl + V (which is Paste)
if you want a screenshot of only a specific window, make sure that window is in focus and hold down Alt while pressing prt scr