In other words some kind of AU? There are plenty here on the board if you want inspiration.
The first thing you should decide upon is which time era you want to work in. Then you should look into ships of that time era. Then decide on what ships you want your navy to consist of.
And then... well, start designing. One of the best things to do is read through different threads in the beginner subforum here on the 'bucket. That way you'll learn what mistakes not to make, since they'll be demonstrated, noted, and rectified(I include my own works in this
If you want to make calculations for a design, I'd suggest you get Springsharp
And yes, using paint is probably the easiest.