if losing weight is your goal, food is 75% of it. Its a big commitment and hard to get into, but once you do it gets easy. I played college tennis, D-1 level, in the early 90's. I stopped playing right after college, and didn't pick up a racket again till I was 30. In the process I went from about 170 lbs to 210 lbs. I did a whole bunch of stuff in my 30's to try and lose it, but the thing that worked the best was getting with a nutritionist a few years ago and on an eating plan I liked. I am now a big proponent of the Paleo diet. Not saying its for you, but I think it has helped me have energy for tennis.
However, here are my basics:
Count calories, stay under at least 2500. Get yourself down to 2000. Have a lot of little snacks and lean meals at lunch and dinner.
Some habits to get into
No Booze, No sodas. Water only, maybe coffee, but no sugar in the coffee
Drink at minimum a gallon of water each day. 128oz's. Get 24 oz water bottle, keep it at your desk and drink it all day, you'll get there. When you feel hungry, drink more water
Lots of Greens, vegetables, Kale, Collards. Lean chicken or meat for lunch/dinner. No Corn, and I prefer to stay away from pastas
No Chips, or fatty snacks; Candy, dessert, etc. Stay away from sugars.
Eat something with protein right after you workout
And yes, give yourself one day to cheat, gotta keep yourself sane. After awhile you won't need to do this
Get 7-8 hours sleep, skip out on going out unless you absolutely need too
For exercise,
There are a ton of boutique fitness places out there. Some of them are really good and not as expensive as a mainstream gym, who will want to get you on some sort of contract. Not sure where you live or how available they are, but I would recommend getting into some sort of class you can take. For tennis you want to concentrate on Power work with legs, core and shoulders, and then plyo work. Get on a two day a week lifting regime, and then 2 days of cardio, or some variation of that. If planet fitness is your only option, check with one of the trainers to see what sort of classes they run. The classes will at least keep you honest, and having a trainer nearby will help you with technique and form. If not, you'll end up with all sorts of injuries and other problems.
Its truly a life commitment, and a 180 probably from what you have been doing, but you'll feel so much better and I bet within a month you'll feel it on the tennis court.