The first step in ship design, is to have some sort of idea as to what the ship is designed to do. Is it a warship? Is it a civilian ship? It is intended to carry cargo? Is it fast? Heavily armoured or heavily armed?

You should try and determine why the ship was constructed. If another ship does the same sort of role, then why build a variant?

On a personal note, I think this is one of the most important parts of ship design. It is terribly easy to select a hull size and load it with weapons, but that gets all the flavour out of the ship. Constructing an SSD is easy. Fleshing it out with a rationale is much harder, but much more rewarding.