If you've got clicky noises in your joints, you need strength training more than the average person! The idea is to make your surrounding muslces strong enough that they take the weight and pressure off your joints. All your joints should do is bend; your muscles should be absorbing the force.
You might want to try some weight machines, instead of free weights. These tend to be a lot easier on your joints, taking pressure off the joint itself by transferring some of it to the mechanics of the weight machine. Poke around your gym and see what machines they have. Also, gym employees or the trainer there can show you how to do the lift properly, so your joints don't get too stressed out.
If you absolutely don't want to do weight lifting, all you need to do is add some type of "resistance" to the activity you're doing. If you're on a bike, do hills; if you're running, add some sprints (although with bad knees, you probably ain't running). You can also get in the pool, because moving your limbs through water is more resistant than moving them through air.
Intervals are also a great way to add strength, as they super-tax the muscles in a short period of time. So maybe try a spinning class.
Good luck to you!