Buy a small container of spackeling compound at any hardware store.
Force the compound into the cracks with a putty knife and scrape it smooth.
After drying, sand the area with 180 grit sandpaper, apply a second filler coat if necessary and prime and paint.
If you think that the cracks aren't done moving yet, you might try DAP latex sealing compound. It comes in a RED tube and you can squeeze it into the crack and wipe it smooth with a wet sponge.
Spackeling compound is the preferred material.