How to make my immune system stronger?

My immune system is generally really strong. I barely ever get sick. I get mild colds in the winter (but doesnt everyone!) and i get stomach viruses like once every two or three years usually. 2 weeks ago I had a bad flu for a week, and I honestly can't ever remember having it before! kind of in the middle of the night last night I started feeling unwell, and I woke up with a bad cold, and have been feeling headachey and a bit nauseous since then. I have a slight temperature, but not that high, I just feel warm. I'm a 14 year old girl, im a vegetarian, I currently dance for about 8 hours a week and I do gymnastics and cheer for 3 or 4 hours. I honestly don't eat that many vegetables or fruits but I rarely eat junk food, and I eat lots of lentils, tofu and stir fries. I take fish oil omega vitamins but lately I've been forgetting. what can I do to improve my immune system?? how much fruit and veg should I be eating ? thanks!