I'm over weight and my knees havin been bothering me im only 24...How can I still start to exercise with this problem or any suggestions on a exercise?

Aqua therapy. Pool based therapy is a good place to start. The buoyancy of the water unweights the joints. The muscles work against the resistance of the water to build strength and burn calories. If you don't swim use a life vest in the shallow part of the pool.
Low Impact. Start with things like swimming, stationary or regular bike, elliptical trainers, etc... All of these offer very little impact and force through the knees. Also watch your diet very closely. Reduce carbs and fats, and increase lean proteins. Another great rule is don't drink calories. If your knees still hurt with low impact exercise see your doc for an evaluation.