How to maintain a good diet durig the days going to gym?

Get good info: I'm very glad you're doing fitness. An athlete's diet contains plenty of protein from whatever source (many gym types purchase very inexpensive protein powder), and avoid foods that are greasy (leave a grease spot on a napkin), salty, and/or sugary (folks tend to overeat these). Learn to tell true hunger from appetite & be comfortable with some mild hunger; it will keep you trim. Good luck.