I'm a beginner and I don't know any openings - what are the best openings for beginners?
I'm going to ignore the "simple" openings and recommend to you good openings for beginners.
It is pretty common for chess instructors to recommend
1 e4 openings to beginners because they tend to be more dynamic and therefore helps beginners improve in their tactical ability.
My following recommendations are in the spirit of fostering tactical and attacking ability.
1e4 e5, I recommend the Italian Game because you will often get the Fried Liver Attack which will help your tactical ability as well as win you many games.
1e4 c5, you could play the open sicilian though it has a lot of theory if you choose to stick to it. I recommend the Smith Morra Gambit which may not be completely sound, but it will give you great attacking chances.
As Black, you will face
1d4 most often.
e5has a lot of theory and white can play many things, so I would recommend either the caro kann or french defense.
Against d4, the King's Indian Defense or the Benoni. Those two openings are pretty much the only way for black ensure a dynamic and tactical game against d4. You could try the Dutch defense, but I find it less tactical and difficult for beginners to play with.
There are easy openings, then there are the ones you're going to see. Your question implies you are a near-beginner. Even so, by now, you have seen:
The Ruy Lopez. This is probably THE kingside opening to learn. You'll need it on both sides of the board.
The Queen's Gambit. This is THE queenside opening. Again, you'll need it on both sides of the board.
The Sicilian. Selected by Black in response to e4. You don't have to play it but you better be ready for it.
The Fried Liver Attack. You'll see it in blitz. Be ready. As black you'll learn to hate the f7 pawn. He's a weakling.
That being said, don't try to memorize tons of moves. This is a waste of time. I recommend you get familiar with the first few moves and major variations and learn what the variations are about. Thereafter, stick with good opening practices such as developing pieces and castling. Because as sure as I type this, you're going to see someone play something weird on move 3 and you're going to have to deal with it.