To start with you'll want either the basic rules or Player's Handbook. The basic rules are free on DnD Beyond (that's the easiest place to find them, but you can also just Google it). Personally, I say that if you're doing 5e, you should have the PHB or some way to access the player options in there

You'll want to go through the character creation section, skill checks, combat and spellcasting. That should be enough to start. Next, I highly recommend making a character. By making a character you'll learn well enough to teach your players.

And finally, nothing has been more useful to me than Matt Colville's running the game playlist. You only need the first few videos to get started, the rest is optional advice and things he likes to do