And eat vegetables and drink milk and so much water.
There is no 1 specific diet you need to be on while hitting the gym.
But if you are going to be lifting weights and focusing at all on building muscle, you'll want to have a diet high in protein. Ensure you're getting 1 gram of protein per pound of bodyweight.
Aside from that, you could eat at around maintenance to do a body re-comp (add muscle while losing fat), eat above maintenance to build muscle, eat below maintenance to lose fat.
What is maintenance? It's: Bodyweight in lbs x 15, which is the amount of calories your body needs to maintain it's current weight.
Depends on your goals. If you're looking to put on muscle, burn fat and put on muscle.
A perfect diet for both these goals would be Oats with blueberries in the morning, a small helping of dry fruits mid day. A good high in protein lunch with brown carbs and chicken or fish as your protein, an apple in the evening, and again a good high in protein dinner with half the carbs you had for lunch.
Stay away from flour/maida/ white bread products. Stick to brown cards: Brown rice and brown bread are good options. Use olive oil over others. High amount of veggies to increase fiber.
Drink a lot of water to increase metabolism.