How to finish dirt floor in a basement?
You need to see if you can first remove the dirt. The depth of the footers will determine that. If you can remove dirt, you'd want to remove enough dirt so that you can put down a gravel base, compact it, then pour a new slab. 

If you can't remove the dirt, you might be able to pour a raised concrete slab.

If you want to go super-cheap, lay down and seal a plastic barrier above the dirt just as you'd do in a crawl space.