Player says what they want to do.

DM decides how difficult it is from trivial to impossible. If it’s possible and not trivial the DM sets a difficulty and tells the player what ability and/or skill Bonus to use.

Trivial tasks always succeed, impossible tasks always fail.

Player makes the roll and calculates the outcome.

DM determines the outcome and tells players what happens.

That’s the base of dnd. The other rules are all just for determining or changing difficulties, as well as changing what’s possible, and what’s trivial. And to create an order of operations.

The basic rules are available for free on the WOTC website: https://media.wizards.com/2018/dnd/downloads/DnD_BasicRules_2018.pdf

Besides reading the Basic Rules, which someone else has already linked, I suggest you check out a game on Youtube. Note that usually these aren’t an accurate representation of how most games actually go- Critical Role for instance is comprised entirely of experienced voice actors- but it’ll give you an impression of how the game runs.

Just don’t base your expectations of your players/dm on them.

Basically, you need a Dungeon Master who will be the story teller. This person is responsible for setting the stage for the story and guiding the adventurers through their quest. The DM also sets up the encounters with monsters. The DM must know each monster, the Hit Points, the Attack Power, special moves, etc for each monster.

The adventures will listen to the opening sequence of the story by the DM and then they will make decisions on what to do next. The DM will guide the players to what they “should” do, but the DM doesn’t “force” them to do anything.

The DM must be able to think on their toes, for players will always try to do something the DM didn’t think of…they must be able to quickly act on a situation. Speaking of quickly act, the DM can take their time to figure out what to say next, just don’t take like 10-15 minutes, or everyone will get bored.

For the players, try to stick to the story as much as you can, don’t go running off and trying to fuck chickens (yes I had a player who wanted to do that all the time).

The players must keep track of their health, inventory and skills. They must know how and when to use certain skills.

I could go on and on, but I see others are commenting now and probably discouraging based on your question, so I will keep this short and hope this is what you are looking for.

Oh, and HAVE FUN, that is the most important part!

Why not read the basic rules?

https://dnd.wizards.com/what-is-dnd/basic-rules

https://www.dndbeyond.com/sources/basic-rules

The simplest explanation: roll to hit, then roll for damage. Or roll a save if it doesn’t need to hit. Or roll a skill check if you’re trying to attempt something out of combat.

That’s about it. You can find the details in the rulebooks.