Walking, elliptical or swimming...those are all low impact on the joints. A lot of folks like the Walk Away the Pounds videos, which can be done at home indoors, and you don't need a treadmill, just a tv and player and a little floor space. If you join a gym, you could get with a trainer and they'd design a program for you to get started with, if working out in a gym appeals to you.
I think the most important thing, for me, has been picking things I really enjoy doing or have always wanted to try. I never could stick to exercising when I chose things that seemed like a "workout", just for the sake of working out, if that makes any sense.
There are many, many ways to get your "cardio", so just keep that in mind, and try to pick from things that will be interesting and enjoyable for YOU, not what someone else tells you is "best". The best exercise, like the best diet, is whatever you will stick with!
If you try something, and later find it's not a good fit or it gets boring, just replace it with something else. It might help you at first to make a list of activities you can do already, that you enjoy, and also things you've always wanted to try, even if you feel they're too hard right now. It doesn't take long for your fitness level to improve, really, so things that seem out of reach now will soon be very possible!
Best of luck to you, and congrats on your weight loss so far!