My advice differs from the weight trainers. I'd focus on cardio if you want to lose weight and be able to play a ball game without getting tired.
High protein diets in mice have shown a reduced lifespan (from 900 days to 700 days). Yes, you aren't a rat, but there have been no clinical studies with humans as it would take many, many years to conduct. Make what you want to of that study.
High fat, low carb diets (like Atkins, Zone and South Beach) have been warned as causing stress to the kidneys and increase the risk of liver disorders, gout, coronary heart disease, diabetes, stroke, and severl types of cancer. (There are some pretty heavy orgainizations backing that statement)
According to the National Weight Registry, which has tracked long-term sucessful dieters since 1996, 99 percent of the people who enjoy lasting fat loss (more then 70 pounds lost and kept it off for over six years) don't use ketogenic or high-protein, low carb diets. They use high-carb diets.
If you want to lose weight and keep it off, limit your calories, not carbs. It's your total intake of calories, not carbs, that controls your weight-loss density, your risk of disease, and how long you will live. All of the positive weight-loss results of high-fat, low-carb diets you read about in the media result from consuming fewer calories and not from eating fewer carbs.
Aerobic exercise promotes cardiovascular fitness and helps lower your blood cholesterol score, blood sugar levels, and triglyceride levels (blood fats). It promotes a healthy skeleton and lifelong weight control, and it helps avert diabetes and many types of cancer.
*Everything above is taken out of the first chapter of "Eat to Win for the 21st Century" by Robert Haas, MS. If you're serious about healthy weight loss and in creasing your stamina I would go out and buy this book. Everything in it is from proven clinical studies and has shown long term success. It will help you plan a better diet and an exercise schedule.
You will lose weight by burning calories, and the fastest way to do that is with aerobic exercise. (cardio) If you're overweight running might not be a good idea since it will be hard on your knees and you'll likely get injured or hate it so much you quit. Start by walking more. Try swimming. Swimming, in my opinion, is the best exercise you can do. It's low-impact and you'll burn more calories for a given time period.
I'm not over-weight and never have been since freshman year in high school. That's when I started doing a lot of cardio for sports. I fell off the wagon when I went to college but I recently got back to it. In the last month I have lost abuot 10 lbs and gained a lot of stamina back. That may not sound like a lot to you, but I didn't have that much extra to lose so the percentage of that 8lbs compared to my entire weight is great.
Summary: Buy some books, eat better (not one of these fad diets), and do at least 5 hours a week of some type of cardio activity. (walking counts)