Did the main board no come with cables? What cables you need will depend on what components you have; i.e. hard drive model, dvd drive model, main board make and model (aka "system board" or "mother board") you have.
At a guess I would say you want a
SATA cable and an
IDE cable. Note I just picked two products (that are probably what you want) as
examples -- they might not be the best choice and they assume you don't have any special requirements.
You should first check that the cables didn't come with the main board -- they normally do. Then if you're sure you need to buy them, post some more technical information about the hard drive, dvd-drive and main board -- that way someone can help you find something that will less likely to be incompatible with the stuff you have already.