Which cat did you have first? This is probably a territory dispute if they were brought home at different times. If you can, give both cats a place where they can retreat to that the other cat can't go...like if you know one likes to sleep in a particular area, discourage the other cat from going to the same place. Give them treats when they are near each other so they learn to associate each other with good things. If you can provide other things that will help them avoid each other, like a high shelf they can use to travel around the house to get to food and litter box, that is also helpful. Providing at least 2 litter boxes in different areas until they get along can ease tensions as well...just make sure that they are in open areas so that neither cat feels the need to defend the box from the other. Multiple food and water stations can also help avoid confrontations.