OK I took some classes for fitness, since I'm going to start as well, and the reasons that your knees and back are hurting is because you don't streatch here is what you do for your knees:
Do a lot of squats, buts since your knees are bad, I recommend doing some stairs, just go up and down from them, after your get good(3 months or so) start doing squats, you do squats by sitting down, but you don't actually sit on anything you just pretend, and if its too easy for you...
for guys you usually put a bar with weights on your shoulders, and for girls your get free weights on your arms(just hold them on your side), not too heavy weights either, I recommend 20-30 pound weights.
And for your back, do some stretching, stand streight and reach for your toes (without bending your knees), go as far as you can, if the back of your knees start to hurt , don't go any further, and remember that its not suppost to hurt and you are over doing yourself.
Hope this helps.