Any type of exercise/ physical activity will work - but ONLY if you are burning more calories than you are eating. It doesn't matter if you train with olympic sprinters if you're eating more calories than you burn in a day, overtime that will cause you to gain weight.
Assuming you are burning more calories than you consume, it will take serious time before you see results with any workout. Consistency is key - it takes quite a while for the calorie deficit to actually make a visible difference.
In order to see results you must find a routine - activity and nutrition - that you can committ to for the long term. You must track the amount of calories you consume and the amount you burn. You can do this online; try searching for calorie calculators. You can also find websites that take your vitals (weight, height, activity level, goals, etc.) and calculate for you the calories you should aim to eat per day to reach your goal. Best bet, if you can afford it, is to hire a nutrition coach and/ or a personal trainer. They can take real measurements (true body fat %, etc) and give you a more accurate plan than any website.
Bottom line - eat fewer calories than you use in a day, and you will lose weight. With your physical limitations (knees), you probably won't be able to find an exercise that will burn major calories, so most of your plan will have to involve cutting back the calories that you eat. Even then, you probably won't see more that 2-3 pounds lost per month.
