If you go see a dietician, they'll know how much you need roughly. In the meantime you can think about what kinds of foods you like / find tasty:
Just some ideas for you for inspiration, based on what you're used to eating:
Breakfast:
- big bowl of yoghurt with granola and fruit.
- Oat meal with peanut butter and banana.
- Greek yoghurt with a granola bar
Snack
- Banana
- Granola bar
- bag of nuts
- bag of dried fruits
- cheese strings
Lunch
- 1 or more sandwiches. Possible stuff to put on it can be (a combo of) meats, vegetables, spreads (peanut butter or something else), cheese
- A yoghurt on the side
- A drink yoghurt on the side
- some fruit
- smoothie with oatmeal
You could also mix it up by taking sth completely different. For example you could ask your mom to make extra dinner the day before so you can have a portion for lunch the next day.
- quiche
- pasta with meat and veggies
- Rice with meat and veggies
Etc
Snack:
- same stuff as earlier are options but I'm adding some more ideas
- something just for fun like cookies or chips/crisps.
- dried jerky
But yeah, these are just some ideas you can start thinking about and talk about with a dietician. I think you can also take this as an opportunity to try out tons of tasty new things