I guess I would need to know what finish you are looking for? Glossy, clean, rustic etc.
If you are looking to bring it back to where it once was you should check out some masonry specific products from a company called Prosoco. They sell a line of cleaners and sealers that just do a great job on old brick. Whatever you do, I would start with a good cleaning but you may also need to strip off an old finish first.
Please be careful in using these items since they are designed for professional use and can be very caustic…but they really do work.