Maria
It's up to you, you can choose to eat in the lounge before you board or eat on-board (or even both). I usually do both [:w]
You will be asked on board what you would like to order from the menu, if you dont specify, you will be served along with everyone else. You can choose to eat later in the flight if you wish too but you will need to specify this. You can order anything from the menu (and in any order you like as well) You dont even need to have the set starter, main and desert if you dont want.
It's your choice what you eat, when your eat it and even where you eat it (at your seat on your own, with a friend or even sat at the bar)

Regards