Tai Chi is great for increasing range of motion in all your joints. A gentle form of yoga (or any gentle stretching) would also help with maintaining and/or increasing flexibility, which is invaluable for joint problems. At the same time, any kind of stretching would help your muscles stay long and lean. Shortened muscles tend to go into spasm more readily, causing more pain and discomfort.
Anything performed in water would be helpful to cushion your weight-bearing joints (both hips and knees are weight-bearing). Fibromyalgia sufferers can get relief from doing gentle exercises in warm (not cold) water.
If you're open-minded and willing to try alternative therapy, you may even consider magnetic mattresses (or mattress liners which are less expensive) that have been proved in a study to relieve pain in fibromyalgia sufferers. If your pain is confined to your knees and hips, and you don't want to spend too much, you could try a magnet wrap.
Finally, you may want to question if the diagnosis of arthritis is really accurate, esp. in someone as young as you are. Arthritis tends to be a catch-all diagnosis when the medical profession really means they have no idea what's causing the pain (i.e. no obvious structural/physical cause found). Often, chronic pain points to an emotional component that needs to be addressed. I'm in the process of putting together a hub on chronic pain. Will let you know when it's done.
All the best!