Good for you that you are looking to get more fit.
Check out the following site for advise on developing an overall approach to tennis training:
Sports Fitness Advisor Tennis Training Section http://www.sport-fitness-advisor.com/
Particularly good is the following section:
The Elite Approach to Tennis Strength Training http://www.sport-fitness-advisor.com...-training.html
Take advantage of the stair climbers, ellipticals and exercise bikes that will be easier on your joints until your weight comes down.
Eventually you will want to work on more speed training, but just getting a food aerobic base on the above devices would be a great start.
You probably already know it is much more effective to eat less to lose weight than to try to exercise it off. Just a little straying on junk food, fast food and high calorie drinks can undo your great workout in the gym.
Hope this helps.