Chum, there is a button precisely for taking a screenshot on nearly all keyboards.
It will be labelled 'Print Screen' or 'Prnt Scrn' or something like that. Press that to take a screen shot. Then without doing *anything* else open up mspaint (as you have done just before). Then do a paste - either by using the Edit>>paste menu option or by holding down CTRL+V.
What might be confusing you is that on a laptop the 'Print Screen' button is often an annoying secondary function so you may need to press Fn+'PRINT SCREEN' to get it to work.
You can do this, I believe in you! You're a winner!
By remembering a complex series of codes, programmes and windows!!!
The only complicated bit is that print screen copies to the clipboard and not directly to a file (and perhaps the use of a function key - typically indicated with a different coloured print on the key itself)
Once it's on the clipboard, the next steps should conceptually be understood by any Windows user. If not, it's time to read some material targetting beginners.
Computers are complex beasts, and the OS + shell software can not do everything for the user. And whilst HCI is always progressing to make things more intuitive, people must still be prepared to put the effort in to learn certain fundamentals before beggining.