I plan on using wood to level out my basment floor and put down a plywood sub floor. What is the best way to know how to cut the suport beams so they are level?

Be sure to use pressure treated tongue and groove plywood and furring. I would use a cheap laser level and place it on your high spot. Level it and then rotate around the room. It will show you how thick to cut your furring for that spot. I'd cover concrete with 6 mil plastic or paint it with a masonary sealer that blocks all moisture.

the only thing i see to add to that is maybe use some foam insulation board between the joist.
as far as furring stips ive used 2x4 and just ran edge ways them level at both ends and then pull a string at both ends, and then a few in the middle, and where the i need to raise the 2xs we just cut some treated shims, till we got the right heigth
this all depends on how much room you have to work with,
if you don't have a lot then using sleeps, 3/4 treated flat ways and nail or tapcom screws
definitely use some plastic.