Much of the tennis advice above is good.
The most important thing in your life right now is to get healthy. You will have the onset of diabetes within the next few years and heart problems soon after that.
The weight will make you less and less active and so everything will get worse.
If you can use tennis as the motivation to get a healthy and strong body, that would be terriffic. If you can take the emotions out of it and analytically look at your food and exercise objectively as fuel and body strengthening, you will be on the right path.