I had a few minor problems when I first got 2 cats... so getting a dog and a cat together could be tough.
A few things I noticed is that you don't want to give either animal too much attention... or else one of them might get jealous and then possibly angry.
Also getting them in the same house for a few days (but in separate rooms) could help... they might get used to each others smells. I would then start to introduce things that the cat slept on to the dog (so he can get used to the smell even more), and vice-versa. Eventually I would bring them both into the same room, with the dog tied up at one end, and just let them see each other, but not get close enough to touch. All this stuff should be very gradual... over the course of many days.
In few cases are you going to get 2 animals to get along in just a day or two.