Throwing new cats together just isn't a good idea. The cat that was there first can experience jealousy or territorial behavior and therefore will act aggressively.
First off, keep each cat in a separate room. Switch both cat's bedding every day- that way they get used to each other's scent.
After about a week, start allowing "supervised" visits. Let the cats interact, but only while you're around to watch them. Your older cat can harm the little kitten if they're left to their own devices.
Feliway is a product available in most pet stores or you can probably get it online too. It's a cat appeasing pheromone spray, totally odorless to us. If your cats associate each other with the way this product makes them feel, they'll be less nervous and be more likely to get along... so it might be worth getting.
When they're out together full time, remember that the rule of thumb is one litter box per cat, plus one box- so three boxes. Following this rule can help curb territorial behaviors like urinating outside the box which is another problem I had with my two cats.... prevent that part before it happens by getting a few extra litter boxes.
There may always be a little bit of rivalry between your cats, but if you take it slow and introduce them over a period of time, they'll learn to get along.
This post was edited by Sozzabell on Mar 22 2012 05:38am