Re:
google spoke calculator and enter your rim and hub and it will tell you what length spokes to get - then just ebay the spokes.
then assemble the spokes into the hub and rim but dont tension the wheel. safest option for you first build is a 3 cross pattern.
then take the wheel into you LBS and ask them to dish and true it - expect to pay £15 for this.
I have done 5 wheels like this now and they are all still going strong.