Well, first you have to have a good idea of what you want to make. This is your next project, a comfy little sloop, as shown in the following screenshot:

You will need something to tell you what size the object you are building will be. There are many ways to get the measurements you need.

Quite often you can find plans on the net on sites like http://www.boatdesign.net/plans/index.htm. You could also measure the object itself. Once, to build a model of an electric guitar, I went to the Carvin guitar factory and took my caliper micrometer and rulers to one of their Ultra V guitars and made some accurate drawings to guide my modeling.

In the plans for the sloop, as shown in the next screenshot, you can see a top view, a side view, and to the very right, there is the number 6 between two dashes to indicate the scale of the drawing. In this case the lines above and below the 6 show the scaling for six Blender units. I decided that a front view was not necessary, because...