Why Nutrition is important
Making sure you are eating correctly is more important and more beneficial than what any workout can do for you. Making sure you get the right amount of protein and calories as well as eating healthy foods is really important.
Protein
Protein is one of the most important nutrients for building muscle. It gives your body energy and provides your body with amino acids to allow your body to repair the muscles tears caused from working out. There a lot of foods that are good and high in protein like lean meats like chicken or beef as well as fish, eggs, and greek yogurt. there's of foods high in protein you just need to make sure you are reaching your proper protein limit for the day.
Calories
The amount of calories you should consume varies from person to person and depends on your scenario. If you are trying to gain weight and build muscle than you need to be eating more than your maintenance calories. If you want to lose weight than you need to be eating less calories than you normally eat. Regardless of the calories you need you should be filling them with mostly healthy foods that provide you with the required nutrients for the day.