First you need to determine the cause of the crack. After that issue is resolved, you can patch the crack with drywall mud and smooth with a damp sponge, then paint. If it's very bad, you may have to patch or replace drywall as well.
How badly cracked? Plaster, drywall? Seams or what is cracked? A photo would help.
Hi. If is plaster and they are hairline open them up because just skimming them over with plaster and they'll probably crack again. Fill but not completely. Allow that to dry, then finish off. Sand smooth and prime, then paint. Just painting and the paint will be lighter when dry. Good luck!