I need help with screenshots!

Idk how to take a screenshot in league. I really want to know how. I tried pressing Fn and Print Screen at the same time in League but it ended up being a black picture when i pasted it in Paint. I also tried F12 but it did nothing. I went to Riot Games> League of Legends to try and find a screenshots folder. When i went to that file, there was Config, Logs, RADS and a bunch if random stuff, no screenshot folder. Please help me and thank you.

I had to repost cause i posted this in the wrong category. Oops xD