ShenVal18 is correct (as always) - you don’t have to worry about roommate yet. Just pay your matriculation fee and that will give you your place in line for room selection.
You will have until early June to find a roommate - then when your time comes to select room you can pull the roommate in with you (meaning even if roommate accepts months from now - you still pull him in at your early time for selection). Great benefit of being Early Decision is you will get a very early time slot for picking a room.