Calculating speed, weight, power, stability requires much effort unfortunately.
There is no easy sum to quickly calculate one's weight. The fact a whole education is dedicated to it, makes one unable to simply explain it within one post. But for a first design, I wouldn't bother much with these details.
If your ship is E.G. 250 meters long, I would suggest to look up information on other vessels of similar size and use the average numbers.
Most of the time, vessels of similar size feature other similar aspects such as draft, beam, power usage, tonnage etc. Like if you notice 3 ships have a draft of 9 meters. 2 of 10 meters and 1 of 8 meters, you know you are making a realistic drawing somewhere within that range. So a draft of 12 meters or only 5 would be unrealistic.