What opening should I learn as black?

I'm a beginner to chess and have chose the Réti opening as my main opening for white. I haven't studied any openings as black yet. I was thinking perhaps the King's Indian Defense, and the Modern for e4. Any thoughts or suggestions would be appreciated. 😁

I'd suggest just playing e5 against e4 and d5 against d4, and having a basic idea of what a normal first five or six moves look like for black in the Queen's Gambit Declined, the London System, the Ruy Lopez, the Italian Game, the Four Knights and the Scotch. Note that you don't need to know every possible variation, just have seen enough to have an idea of what normal development looks like.

Come to that, I'd consider playing the white side of one of those openings as white. IMHO it's a good idea as a beginner to play something that immediately fights for the centre in a classical way - openings like the Reti, the KID and the Modern work on more of a counterpunching thing, letting the opponent grab the centre but subtly applying pressure and striking back at the exact right moment. As a beginner, nine times out of ten you won't apply enough pressure or strike back effectively and you'll just get squashed.

@RamblinDave I second this, play 1.e4 as white and agaisnt 1.e4 play e5 and against 1.d4 play d5. Put a pawn in the center, develop your pieces, castle. The ideas in something like King's Indian Defense is too complex and counter-intuitive for a beginner.

Agree with previous: as black 1 e4 e5 and 1 d4 d5, as white 1 e4
You do not need to learn anything. Play 2 pawns in the center, develop knights, bishops, castle, bring rooks to open files, find a good square for your queen.
Play and think carefully about your moves. After you lose a game analyse it and see what you did wrong. Learn from that.

There is no need for you to explore openings right now, all you have to do is castle and develop pieces to most useful squares.

@RamblinDave after 1. f3 if d5 then I instantly go d4, I know if black gets center pawns past the 5th rank it's bad news. I like how flexible the Réti is and how early I can develop and castle kingside but I admit that it and other hyper modern openings may have too complex of ideas for a beginner. That said I'm not intimidated to learn theory. I'll definitely learn some classical openings for white. I may be getting overzealous with the hyper modernism but I just think it's a beautiful way to play.

As for opening with black do you think the French would be a good option? As it immediately advances the pawns on the D and E file?

I would recommend the black lion. 1. e4 d6
2.Nf6 and I will leave you to study the rest

Do 1. d4, d5 2. c4. Very good.

Not the modern or Kings Indian for a beginner.

Try the queens gambit declined as black against d4 and something more solid as black against e4. Personally I am fond of the Caro Kann but maybe learning e4 e5 would be a good place to start.