step 1, go and see someone who knows about knees and follow up suggestions. It's not that poles are a Bad Idea, but if you can get things more better at source that's better, and stronger knees and poles are better than wekaer knees and poles. Having had some knee trouble lately I went to see a physio earlier this week and I've a set of exercises which will hopefully improve matters a lot (in my case an imbalance in my right quad muscles with the inner not as strong as the outer).
Beyond that, poles are sticks, and aren't Rocket Science. You don't need to spend a fortune, but if you spend more than the minimum you may get useful features. Some poles are stronger than others, which is handy if you're particularly large (or in my case, you use them for skiing too). Some are lighter, which is an obvious benefit, and some have more money spent on grips and/or straps: the ergo grips on Pacerpoles have many fans and let you get straight up to speed with poles without much need to develop technique, probably good if your knees are already suffering. If you stick with conventional grip/strap, wider straps in smoother tape will generally be more comfortable.
The locking mechanism bothers some people. Camming levers are a more expensive alternative to the more usual twist locks. They're also a bit heavier, but considered more reliable and are easier to fettle if something goes wrong.
Do practice and get some technique. Waving them at the ground doesn't get weight on them so doesn't help your knees. But it does help them get in the way, and is what at least half the peopleI see with them are doing.
Pete.