Gotta take care of those knees! This article on knee health has a ton of good info about how to avoid knee injuries, and what to do in case you do get injured. It also includes details about how the knees work so you can better understand what's happening when you use them.
If you're wondering how to stengthen your knees the best thing you can do is strengthen all the muscles around them.
One great way to start is with your basic front and back scales. They're incredible for building stability throughout the lower body and the core, and they are no-impact. They look simple. But damn. Try doing a full, slow, smooth set of front and back scales and see how easy they really are(n't).
If you want to take your leg strengthening even further, we have this article. It includes 9 different lower body exercises designed to increase strength, mobility and balance. Another great addition to improving knee stability.