You can play using the awful gamespy arcade tool, which will probably come with the game. Its free to use and probably one of the worst things I have seen since Star Wars Galaxies.
But, thats what is being used. There will be 1v1, 2v2, 3v3, 4v4 skirmish modes, which can be fought on either land or space, together with a Galactic conquest mode which will only be 1v1. There is no monthly fee, although gamespy do charge a monthly fee IF you decide to subscribe which means you wont get adverts thrown at you during the middle of combat, as has happened to me.