There are many resources and tutorials available. Your first stop should be OrbitHangar. Do a search for base building tutorials and tools. Be sure to read the pdfs supplied with Orbiter as well as the documentation in the SDK section.
I would highly recommend Orbiter Base Maker ver. 2.
You can use it to piece together bits and of other addons to create your own (obviously providing appropriate credit to the other authors).
If you want to try your hand at 3d modelling to build custom base pieces, there are many programs available. My personal choices are Blender 3d, But wings3d, anim8tor, gmax are also recommended choices.
There is a complete section on addon development in the Obiter-forum.
Lots of resources and lots of advice are available.
Have fun.
StarLost