While you can measure the dimensions and do the calculations on your own, it's usually easier just to go with a spoke calculator. These are data bases of wheel component measurements that are used to calculate the spoke length you need given the rim, hub and cross pattern you want to use.

I like Damon Rinnard's Spocalc but it's a bit dated. You can add to the data base on your own but the most recent versions of the spreadsheet I've seen are getting old. I also find that it gives values for the spoke length that are 1 to 2 mm longer than needed. URL="http://www.prowheelbuilder.com/spokelengthcalculator/"]Pro Wheel Builders[/URL] has a good calculator that is up to date but doesn't cover old stock like Rinnard's does.

If you are going to build a generic wheel with a Shimano hub, generic spokes and a generic rim, you won't save any money over just buying a wheel. I don't want to discourage you from building a wheel but just realize that you won't save any money doing it. On the other hand, if you want to build a wheel that you just can't buy anywhere like, for example, a Phil Wood hub, 13/15/14 spokes (actually 2.3/1.8/2.0mm) and Velocity Synergy OC rim, then you'll probably have to build it.