You already have the tennis shoes, tennis balls and a racket (racquet), some suitable sportswear, energy drink and a banana?
If not you can visit sports shops on our sponsor pages to get kitted up with sporting goods. Most serious players use Babolat rackets. Personally, I get on well with Head and Wilson rackets. It's a personal preference thing. For balls, if you play regularly, you should only use pressurised balls. These are the real thing. There are longer-life unpressurised balls on the market. They are a good idea if you only play infrequently as the pressurised ones lose pressure over time. They behave a little differently so it's not a good idea to mix. Some good brands of balls include Slazenger, Tretorn, Head, Wilson, Babolat.
Look after your racket and get it re-strung (restrung) if the strings are loose. You can buy new grips or overgrips so there's no excuse for the racket to go flying out of your hand.

You're already fit? If not, join some other local sports clubs or use a gym. There's nothing better than multi-sport training.
Diet is important. It just needs to be sensible and balanced. Healthy home-cooked food is best. Some fitness fanatics use food supplements. Their benefit is doubtful though.
The final ingredient is coaching. Tennis coaching is available through most clubs or look online to find freelance qualified tennis coaches or personal trainers. Regular coaching can lift your play to a new level.

NSL Tennis: the automated nationwide competition system by Ben Friman, Bedford UK.

NSLTennis could be what you're looking for. It is a series of local leagues all over the country for players of all abilities. It is free to join.