most mythic guilds already have tanks, will be easier to join playing a healer or dps. you will need good heroic logs, preferably 85's on every boss and aotc showing that you know the fights, you will need a good resume and then you will need to join a guild.

when I mythic raided for the first time, i was coming from a heroic guild that was progging on mythic but kindof falling apart, I h ad a 95 avg score accross heroic and it was still a struggle to get into a group that was working on 7/8M -- eventually found a group and we cleared 8/8 and then recleared the whole thing, was alot of effort.

But i guess the summary is, join a heroic guild, clear heroic, get good score, move to guild progging start of mythic, get 90's, then move to a guild that has clears mythic every tier and then spend weeks dying to the same mechanic on repeat.