Hi, Val. How big are the cracks? Are they just hairlines or do they have any width? Does the plaster appear or feel like it has a big "bubble" that is a foot or more in any direction or just small surface bumps? How old is the house?
There is a good thread a little down the board where Steve S asked about painting prep. Let's get your plaster fixed first, but then you will want to check out that thread.