Well, the first thing to do is to start reading about real designs/never weres of whatever ship type you want to design.
Secondly(What I did), read through the beginner, alternate universe, and personal designs forum. Especially by reading the beginners forum you'll get to know what mistakes people often do in the beginning, and how you won't repeat them(I lurked on the 'bucket for months before starting to post for myself).
And thirdly... well, start designing. I'd recommend getting a program like springsharp if you want to do some calculations, but it's not necessary.
Fourthly, and maybe the most important: listen to the advice provided by other members. Many beginners ask for advice, but ignore it when it doesn't align with their ideal view of a ship, even if it means the design wouldn't work.
This might seem like a big piece of work, but it's really how most of us started. Good luck.
