So I just hit 345 IL and want to start mythics. Ive read to start my own group would be best. Now my question is do I have to run to the entrance? Also what one?

I'm pretty new to wow as well
But yes I believe you and 1 friend have to run to the entrance in order to summon the rest

As for which one I think it depends on which mythic you wanna do

Take in mind I think you have to do story line to unlock them

I could be totally wrong on everything

You only need to do the quest line for two of them, all the others are already available to do(forget the names of the dungeons).
As for the summoning part yes you will need yourself and one other there to help. Your ilvl is lower for the current game progression but if you post your key and say "fast run" people will join and come speed through it and even give you gear as well.

You must unlock boralus and king rest
U can do all of em without those two
2 players is a must to start summoning

Boralus and Kings Rest were unlocked patches ago. Just run to the enterance, make sure your difficulty is at Mythic, and go inside