How to join grand company?

[Pintsized]

It's not your character level alone, you need to play the Main Story up to and including the Ifrit trial (which is a level 20 quest hence the level requirement you're aware of), after that you'll begin a short Main Story quest chain that will get you to the point of being able to select which GC you want to join.

In order to unlock jobs you'll need to complete another couple of Main Story quests once you've joined the GC (specifically one called "Sylph Management") .. welcome to FFXIV, a game where most content is gated by the need to follow the tramlines of the Main Story, a 'sandbox' MMO it certainly is not