I just got RPG maker 3. I made a town to start out in with but cant find a way to make streets. Is there a way to customize the ground in towns? road, brick, grass, ect.

There are various objects you can place on the ground, including grass patches, but you cannot edit the actual ground.

I agree with Draygone. You can suggest a road by lining it with trees or grass patches (and some towns already have dirt roads but the town texture is set.

EDIT: You can make roads in Fields though, but you can't place any buildings or characters in fields but only dungeon or town representations. These represenations are so that you can stand in a Field look around and see which direction one town is relative to another or to a dungeon entrance. You can also set it up so that the chacter can enter the town or dungeon directly from the field map.