Q: im just about to start weight training. Obviously i'll have to train my legs along with the rest of my body but im concerned about putting excess load on my knees as i weigh 220 pounds. what excercises would be best?

A: It's always a very good idea to consult with your doctor prior to starting any training program. If you have knee problems in general such as chronic joint pain or ligament tears in the past, then consulting your doctor is an absolute requirement. Assuming you do not have any existing knee problems and you doctor cleared you to start weight training, the following things will ensure that your weight training, including leg training, is safe and effective:

• Learn how to perform each exercise in good form and ALWAYS FOLLOW IT. This becomes especially important for exercises like squats or deadlifts where bad form can easily lead to injuries. Buy an exercise guide (a good one is 'Strength Training Anatomy' by Delavier) to learn how to properly perform different exercises. Ask the trainers in your gym to work with you on your form.
• Start with very light weights or no weights at all until you are completely comfortable with the exercise. For example with squats, you can practice at home using a broomstick.
• Do not push yourself to the point of muscular failure (total exhaustion) unless you have an experienced spotter and/or proper equipment. Doing freestanding barbell squat to failure or barbell bench press to failure can be suicidal. It's much better to stop a rep or two short of failure than risk serious injuries, even if you have a spotter. I would not recommend doing the squat or deadlift to failure under any circumstances.
• Never ever sacrifice good form on an exercise in order to lift a little more weight. Goes along with not pushing to absolute failure when it's not safe.
• Do not overdo it. Many people believe that more is always better and spend hours in the gym every day, pushing themselves way past their limits. For 10% of people this will give them spectacular results. For the other 90%, it will lead to overtraining, injuries, and lack of progress. I would say 5 hours per week and somewhere around 100 sets for the whole body per week should be the absolute maximum limit of weight training for most people. As always, I recommend changing your programs every couple of months and following low-volume (less than 2 hours/week, 30 sets/week total) ones as well as medium-volume and high-volume (as described above) at different times.
• Never push yourself through pain other than regular muscle tiredness during a heavy set. When doing weight training at proper intensity, the last several reps of any exercise will be hard and even somewhat painful. But that pain should be confined to muscle tiredness. You should never ever experience any other kind of pain during exercising. Stop exercising at the first hint of joint or other non-muscle tiredness pain and see your doctor. Do not resume exercising that muscle/joint until the pain disappears for good.
• Work on your flexibility by stretching.

As you can see most of these are fairly common-sense suggestions.If you have no prior knee problems and follow general safe exercising guidelines you will be fine and eventually you should be able to master the squat, lunges, and leg press all of which should form the core of your leg training. I would recommend avoiding knee extension machines (this) since they do put a lot more stress on your knees than other leg exercises without any special benefit in return. And once again, if you have any concerns, consult with your doctor.