how do i lose weight and do cardio with a hurt knee?!

Exercising with bad knees can be a very painful experience. Performing
cardiovascular exercises with bad knees can therefore be very hard to
accomplish, and can even cause significant amounts of damage in the
long run. If you have had a serious knee injury in the past, or are
recovering from some form of knee surgery, you must learn about cardio
workouts for bad knees and include them in your exercise programs.
Doing so can considerably lessen the amount of pain that you feel, and
also chase away the risk of injury, and aggravating the bad knee.
Here are some cardio exercises for bad knees that should be relatively
less painful on the knees.
Cardio Exercises for Bad Knees
It is important for you to remember that you must not bend your knees
excessively, even while performing these knee exercises. If your knees
are being bent at an angle that they are blocking the line of sight to
your toes, this means that they are being bent excessively. This puts
a lot of extra pressure on the kneecap, and any injury or surgery that
you have had on that knee cap is bound to get aggravated.
Here are some of the best exercises for bad
knees that you can carry out to ensure that your knees stay injury
free.
Stepups
To perform this exercise you must use a step bench or a staircase.
What you need to do is place your left foot on top of the step, and
use your calf muscles to push your body up. Slightly touch the step
with your right foot, and then place the right foot back down
immediately. Repeat this exercise with your right foot now. Remember
to keep your knee straight and to keep it in line with your ankle.
Side Lying Leg Lifts
This is widely known as one of the most effective cardio exercises
for bad knees. You need to lie on your left side, with your head
resting in your left arm. Now slowly raise your right leg (which is
parallel to the ground) up in the air, and then slowly lower it as
well. Repeat this exercise for both legs as many times as you want.
Partial Squats
To perform this exercise you must stand a few feet away from a chair
or a bed. Keep your feet shoulder width apart, and keep your knees
straight. Now bend forward using your waist and hip until your
forehead touches the chair or the bed. Use your hands to push you back
to your original stance. Remember to keep your knees straight at all
times, or else these cardio workouts for bad knees will be rendered
ineffective.
Calf Raises
This is an exercise that is very popular for building up the calf
muscles, but it is an extremely useful exercise for bad knees as well.
Stand with your feet hip width apart, and slowly raise both your heels
off the ground simultaneously, so that you are now standing on your
toes. Do not bend your knees, and carry out as many repetitions as you
are comfortable with. You can also use a wall or a chair for balance.
Straight Leg Raises
This is also one of the very popular cardio exercises for bad knees.
Sit with your back against a wall or any other flat surface. Bend your
right knee but keep your right foot firmly on the ground. Now slowly
lift your left leg as high up off the ground as you can. Hold it there
for a few seconds and then gently lower it back to the ground. Repeat
this exercise for your right leg as well.
Hamstring Stretch
Lie flat on the ground on your back. Keep your left leg flat on the
ground and stretch it as straight as possible. Now loop a towel under
your right thigh or your right knee, and pull the leg towards you as
far as you possibly can. Hold the leg in place for a few seconds
before you release the towel, and then repeat this exercise with your
other leg. It is widely accepted that this is one of the best cardio
exercises for bad knees, and helps in strengthening the hamstring as
well.
Short Arc Knee Extensions
The posture for this exercise is similar to that of the straight leg
raises. Sit with your back against a wall or any other flat surface.
Place a basketball under your left knee, and slowly lift it in the air
while simultaneously straightening it. Lift as high as possible and
then gently lower it to the same position as before. Now repeat this
exercise with the right knee.
Exercise Bike
Most people who wish to carry out cardio exercises for bad knees
prefer the exercise bike. This is one of the best cardiovascular
exercises, and the impact on the knees of a person is absolutely
minimal. There is no risk of an accident or falling down as well, and
it burns many calories. This is one of the most favorite tools for
somebody with bad knees or joint problems.
Swimming
Like cycling, swimming too burns calories, builds calories,
strengthens muscles and provides low impact to the knee. It is a great
form of cardiovascular exercises, and it provides many varied benefits
at the same time. For someone suffering from bad knees, this is a
great mode of exercise.

I liked Turbo Jam cardio & strength training b/c they had low impact moves that I could handle okay while still seeing results.

If you've never done it before, I would definitely do the 15-Min Learn and Burn video first b/c the instructor breaks down the moves for you, and tells you how to do them properly while getting the most out of the movement.

Swimming might help you. Also, I have seen one person with an injury use the corner of a pool and do "sprints" Basically, he just ran in place, but propped himself up and used the water for resistance.

you should consult a doctor first. and i suggest you do bikram yoga. it does wonders for injured joints/body pains.