Hi,
If the existing paint is in good condition ie, not flaking, peeling or in generally poor condition then sand with 280-320 grade wet and dry (used wet). An undercoat should then be applied (yacht primer) it is easy to sand to get a smooth finish and allows a better colour when the top coat is applied. If filling is required it should be primed, filled, primed and them undercoated. If the existing paint is flaking then all the old paint should be removed and primed, undercoated and top coated.
Note, if the existing paint system is 2 part then it can be overcoated with either two part or one part but if the existing one part them it cannot be overcoated with two part.-(the chemicals in the two part will reach with the existing one part paint finish). If the hull is dimensionally stable (eg marine ply) then one or two part can be used, of hull is not dimensional stable- like carver or clinker, then one part paint only, as its more flexible. Go to an International paint stockist and pick up a free "boat paint and product guide" all the paint manufacturers do a booklet, they have the same information in, but just say to use there products.
Depending on the finish you are looking for depends on the amount and different grades of sanding and between coat sanding you do.
Make sure to remove all sanding debris out of screw holes etc and go over with thinners and then a tack cloth.