First thing, is to reduce your food intake, not necessarily counting calories or to only eat greens but reduce how much is being ate through-out the day, mainly reducing portion size.
One of the best activites to reduce belly fat and tone your core is skipping (jump rope) but due to your joint issues, skipping may not be ideal; alternatively walking, just normal everyday walking but at a continual and confortable pace through-out;, not power walking, although power walking, if fit enough may achieve the desired results quicker, although, in my opinion not necassary.
Jogging, rowing, cycling, swimming, are all good activites to improve fitness and reduce belly fat but again due to your joint pain, such activities may not be suitable.
The key to all the activities I've mentioned, is to maintain the same pace through-out, thus finishing on the same pace as when started; once the pace slows due to exhaustion, the desired results wont be achieved within a reasonable time
If you're unfamiliar with pace, think of how US Soldiers chant a song to the pace/beat of their marching footsteps; this maintains a continual pace through-out, from start to finish.