where can I start to draw a ship in 3d?

You probably need to find drawing or some kind of reference online first. You probably need to start on the hull first and then the upper superstructure.

There is no easy start.
Download Blender or Sketchup and head on over to youtube.
Andrew Price and Grant Abbitt have tons of excellent beginner vids.
Once you have the basics do a search on how to model a boat or ship in blender.