If your foundation has one crack, chances are there are a few more. But it's not the end of the world. Many people don't take the obvious precautions for keeping water out of their basements, like putting extensions on all of the rain gutter downspouts, and making sure the ground and driveway around and adjacent to the house are pitched away from (instead of towards) the house itself.
Regarding the actual crack(s): If it were mine, I'd try the quick-and-easy way first--apply some polyurethane sealant on all visible cracks (both interior and exterior). To keep the sticky goo in the cracks, you may have to tape the crack faces, then leave small gaps for caulking gun tip access. Start at the bottom, and work your way up the crack. If you feel extravagant, try some Adeka Ultra Seal P-201, a water-swelling one component sealant, instead of the polyurethane (20 bucks a tube, the last time I used it).
If the foregoing doesn't work, then get out the checkbook and pay someone big bucks to excavate around your foundation, installing drain tile or perforated pipe, sealing the exterior walls, and backfilling with pea gravel.