You can plant seed ...
Till add seed, fertalize, WATER the hell out of it until it takes.
There is also this crap they sell that is seed, fertilizer, and water retainer that is useful for smaller areas...
You can overseed, which requires a machine (rent it at homedepot) that breaks open the ground just enough to dispense seed into.
My guess is that sod isn't the way to go, unless you are worried that your topsoil is going to wash away allong with new grass seed.