so i got an 89 probe now i want to add some good sound....i know im going to start from headers but how big of piping should i go to the back and what kind of cat and muffler should i use for a good sound, also anyone got any other good tips for adding a few extra hp?

Well I would go with 2.5 inch pipes. (since you said headers I assume its N/A.) Muffler would be up to your taste just stay away from the cheap ebay ones. Sound too Ricey

go with 2 1/4 pipeing any bigger is a waste and get a magnaflow sounds great