Dizziness & blackouts

I'm 18 years old, 5ft 9in, 10 stone and a smoker. For 4 years I have been suffering from blackouts and dizzy spells. They started 4 years ago when, for no apparent reason, I blacked out, couldn't see or hear anything and fainted. Now I don't get it as serious but everything blacks out and I can't see anything for a couple of seconds, and feel dizzy. It can happen at anytime and I can't find anything to link with it. I have been to my doctor who took some blood tests to see if I was anaemic, but it came back negative. And ever since then he tells me to eat more fruit and veg, which I do anyway. Please help me!