When you can, eat unprocessed (a.k.a. natural, wholesome) food. Delicacies – for example, cheese or chocolate should only be eaten in small portions as a treat. Control your portion size to a sensible dinner plate (i.e. enough but not piled high). Exercise every day in a way you can manage. That can be even be walking for half an hour or an hour.
Which takes us back to the point about school meals – all school meals in Scotland are balanced over a week. Yes you can have fish and chips – just not every day!