Can u play GTA V all multiplayer? or do you have to beat single player first?

Nah you dont have to play single player to unlock multiplayer. It's two different mode.

In single player, you are alone. Cant coop mission, its just you.

If you go in multiplayer, you wont the guns/vehicules that you hace in single. You start with nothing and need to get your own guns/vehicules/house

so you can co op missions to get all that stuff?

yep