If you are new to woodworking, you have to know that fresh wood will tend to crack when it will dry, so better to let dry the wood for some time before starting working on it. Also if you are working on an old wood, keep in mind that wood needs some maintenance from time to time.
Sometimes cracks in the wood can be very annoying, anyway there are different types of them. Some of them can be filled with wood sealer, which will fill the cracks (and it won't be very visible), and if there is a part of wood that is falling apart, I would recommend to use some wood glue.