I'm just wondering, if you want to gain muscle, do you need to watch what you eat?

Eat sensibly: When you're trying to muscle up, try to eat one gram of protein (preferably lean) for each pound you weigh, or 2.2 grams for each kg. A snack of maybe 20 grams of protein after a workout is helpful. Avoid greasy, sugary and salty foods as much as is convenient. Creatine isn't a nutrient but will help you gain mass. I am very glad you're undertaking fitness activities -- make it a lifetime habit.

Healthy weight gain.: You do need to eat a good amount of calories to support a muscle building exercise regimen, but you want to still eat healthy foods so you don't just gain fat. Lean proteins (one gram per pound of body weight per day), complex carbohydrates and fiber.