Please recommend some complementary openings.

I'm learning the Sicilian Defense, but I don't know what to do if White doesn't play e4 . Recommend a few openings for both White and Black, not too many—just enough to handle most positions.

As White, I stick to normal stuff. The typical e4 is where I like to go. If I do a d4 opening, it's usually the Queen's Gambit. I also like to do the Reti or KIA occasionally, as well as the Scotch (e4 e5 / Nf3 Nc6 / d4) and Ponziani (same as Scotch, but with c3 instead of d4).

As Black, my usual response to e4 is either the Sicilian or Caro-Kann. For d4 I usually just go d5, rarely using the KID and occasionally using the Slav (same as the Caro-Kann, only it's played in response to the Queen's Gambit). If I do d5, I like to do the Krause (2. c5; basically the Queen's Gambit for Black) if White's second move of choice is Nf3.

Rarely, people will do 1. c4 as White. This isn't bad, but most people will tell you not to play it as it can be very tricky to play well. If you're Black, the best response to this is probably 1. e5.

Against 1.d4 most practical is to learn queens gambit declined, which means moves d5, e6, Nf6, Be7, castles, and then if possible fight for center with c5. This setup you can play against any closed opening. I white I encourage you to look at four knights Scotch. It teaches you to fight for the center and play in both flanks. Also you should study annotated games of old masters like Morphy, Tarrasch, Capablanca etc. But at the end of the day, at 600 level most of the games are decided by who blunders less or more.

I was learning endgames at first, but I kept getting checkmated in the opening, so I figured I’d better learn some basic opening lines.

If you fall into a opening trap, you just need to analyse it with an engine several times to realise how to avoid that concrete trap and remember it forever. The most important thing in chess is to realise what your opponent wants to do.