Postponing Main Quest?

The main quest consists of 38 missions in a fixed order, that you can basically play whenever your want. Completely postponing even the first two or three missions is not recommended (I don't know if it's even possible), as they are teaching you some basics and introduce you to the game. You also have to complete some of these missions to unlock new weapons and skills, so if you postpone these too long, your will not be able to develop your character. The game permanently let's you know if you have to tackle the next main mission or if you're free to roam around and postpone it, so there's no need for any mission names to look for or to avoid.