Crescent wrench. To hold the spoke in place. Expect some sort of spoke prep to gum things up. Purchase extra spokes just in case.
I had to replace some spokes on a bladed pre-built wheel recently. Had to just cut them off eventually, but the wrench was a lot of help when putting the new spokes on. Get one of these to check the tension, and match it up with the adjacent spokes.
Someone else will probably come along later with better advice, but that's how i did it.