How does M+ work?

I’m very new, I recently max leveled a few characters in BFA for the first time. I’ve been enjoying the game, and Mythic+ has looked very intriguing. Can somebody give me a rundown on how Mythic+ works and where to get started? (For after I level in the new expansion)

Fairly simple, you do a base mythic dungeon (M0) which gives you a keystone for a specific dungeon at a +2. You get a party, go to that dungeon and there will be a font on the ground you insert your key into to start that dungeon as a mythic+2.
This will give it extra affixes (mechanics) as well as extra boss health and damage and a timer.
If you finish within the timer your key will be upgraded, if you fail the timer your key will be depleted and lose a level (to a minimum of +2).
Rewards increase the higher key you do, you only need 1 key for each run.