A couple of questions:
What part of the house is the crack in? Is there more than one crack in the house? How old is the house? How long has the crack been there that you've notice it and has it gotten bigger and wider since the first time you noticed it? Take a look at the outside walls from about the same area that the crack is in, do you see any cracks in the walls if brick or block and is there any cracks in the foundation. If you answer yes to any of this questions; you need to get a contractor out there asap as there is an underlying issue causing the cracks and needs immediate repair work. If answering no to all, then most likely it's just some small settling happening, that has been happening since the day is was built and this is normal.
Repairing: depending on the size, shape and length of crack will determine you plan of action. On a small crack some fiberglass tape and joint compound will do just fine making sure that you feather out each layer of joint compound. It typically is feathered 10 to 12 inches on each side of crack to make it esthetically pleasing to the eye. If a large crack (with out pictures) will involve cutting the damaged area out and replacing with a patch (a piece of sheet rock) and then tape and mud.