The key is in the introduction. Cats aren't like dogs, they need slow introductions that involve separating them at first and letting them get used to each other's scents. There's usually a lot of hissing to start because they're both scared of each other, and it's essential to give your original cat extra love to make sure s/he doesn't think that they now have to compete for love/food/etc.
I would take a gander at The Cat Site (
http://www.thecatsite.com/), there are articles about introducing cats and the forums are very helpful too.