There are so many questions surrounding Mythic dungeons that they have become somewhat of a logjam to progression for some players.  

Reddit has the answers. 

How do I run Mythic+?

Mythic+ is a dungeon that has had a Keystone used in it. You enter on Mythic, and then place a keystone in the pedestal inside.

It causes all enemies in the dungeon to gain more health and damage, and depending on your level, extra abilities to enemies too. In exchange, you receive much higher quality loot, all the way up to ilvl 880.

How do I get keystones?

You get a keystone off your first Mythic or Mythic+ run of the week. It will be a random dungeon, with a level of +2. This is the starting point for Mythic+.

For answers to many more common questions, visit this Mythic Dungeons FAQ.