I don't think it is any trouble for your knees unless you fall on them a lot. If you give it a try don't be expecting an easy time of it. The one thing in common with skiing is: put it on edge, and it will turn. Theoretically, anyway.

I think it is pretty much universal that everyone will crash and burn the first few times they do it. If you don't understand the basic principles - a lesson is probably a good idea. Oh, and get some wrist braces! Something like 95% of all snowboard injuries are related to the wrists.