Sanding is often done to prepare the model for painting.  Any filled seams and sprue attachment pionts may require sanding.  Many folks use fingernail filing boards.  Not the cheap, very rough ones, but the ones that are on a type of foam backing.  These work very well and come in different degrees of coarseness.  You may want to pick up a few different types of these and experiment with them.  I buy these from the cosmetics area in the grocery store.  I watch for them to go on sale and then stock up on them.  As Bagpiper said, this is a very broad subject and there is no one answer.  Just experiment and try different methods and approaches to find what works best for you.  Hope this helps.