the slab need alot of prep attention. The more attention you give to it the less likely you will have those enevitable cracks.
First grade the site. lay down some good sand or gravel. Make the slab 6 inches minimum in thickness and DO install rebar and tie it into the slab of the exterior wall. Drill holes so that the rebar goes into the existing slab about 8 inches or more.
Install good forms, screed to a slight fall so that water will not pond when it gets in. Make sure the concrete is mixed properly and let it dry properly. Install your fasteners in the wet concrete to hold and secure the floor plate studs.