Rod,
You did not mention your experience level.
First you'll need a small foundation in spindle turning and face grain work.
make a connection with an AAW chapter near you.
http://www.woodturner.org/community/chapters/LocalChapters.asp
Then you'll need a plan. the process will be different depending on the lighthouse style you choose. A Chesapeake bay style like Tomas point light or one of the tall cylinders ones light those in New England or the outer Banks.
from the bottom up
1. base Consider a burl with the natural surface to give the illusion of a rocky island. or a wooden disk dyed blue for water.
2. structure a cylinder of wood to scale to match the insulator or spindles for pilings attached to an octagonal glue-up of segments for the thomas point style.
3. the light- your insulator
4. the roof
through the center you will have a hole from the base to the light fixture.
this would be the diameter of threaded lamp rod. the base needs and opening for the nuts to tighten the rod and hole to the side for the cord.
a consideration is the light. I'd suggest the led's because they generate very little heat. you will need to turn or drill a space to accommodate the fixture.
You may also wish to have your light turn, blink, or be focused. this requires a small motor, controller or mirrors these kinds of things can be found at the model railroad store. In fact this is the kind of project they might be happy to help you with.
Finally a wood burner can be used to create the bricks or the weathered boards
have fun,
Al