How do i boost my immune system?

Its like ive always got some sort of bug or something and am feeling poorly. I dont smoke, rarely drink and dont take drugs. Admittedly, my diet is poor- hardly any fruit and veg and alot of fatty foods - takeaways, burgers, chips etc. I sometimes suffer from shortness of breath and the feeling of erflies/aching in my chest. I basically want to boost my immune system as im sick of feeling like crap. Any advice? Im pretty sure its down to my diet but i'd like advice on any vitamin tablets to take and what are the best foods to eat. Any help is much appreciated.