I suggest you go to sheldon brown and read his wheel building stuff.
Spoccalc can estimate the spoke length you need. When I used it, it came out 1 or 2 mm too much. If you have a dremel you can grind off the excess. I had recessed spokes holes so I just left things as they were.
I bought a Park Tensionometer. I trued it on my frame. A truing wheel might be a good idea, but is optional. eBay might have something cheap. I think the tensionometer is really usefull for a first time build.
I bought from lickbike.com and was treated well. Ask them out which spokes to use. It is better if you buy everything from the same supplier. Some suppliers will build the wheel for you for a fee.
