The first step is to get the dog to focus on YOU. This is done by holding a small piece of a very SPECIAL treat close to its nose while it is sitting next to you (no cat present), and bringing the treat up to YOUR face while saying, "LOOK". The dog will follow the treat's movement up to your face, hence LOOKing at YOU. Then it's "GOOD BOY", and he gets the treat. Repeat several times until you say, "LOOK", without luring him to look at you, but he does anyway. Then, "GOOD BOY!" and the treat. PRACTICE, PRACTICE, PRACTICE! Every day. It has to be ingrained into his mind to LOOK at you. Once this is accomplished, you can get him to sit, stay, down, etc. Nothing can happen, as you know, without first getting his attention. Once you are confident you can always get his attention, you can take him (LEASHED) on a walk - with treats handy - and practice teaching him that when he sees a cat (or whatever he might want to chase), he gets a treat for looking at YOU. You'll need to be alert on these walks so you can ANTICIPATE his reaction. It's best if YOU see the cat first, so you can get his attention right away. That way, you'll be set up to succeed.
Has he been to dog school? The more experiences you two have together where he is paying attention to YOU in a social and distracting environment, the better!
This is only the beginning, but definately the most important first step. GOOD LUCK TO YOU! (for the cat's sake!)