I often have a stomach ache when I don't eat a lot during the day, sometimes I even eat only one meal

For a few years now I have always had a stomach ache, in the morning I wake up with a stomach ache and I don't eat, I only drink an orange juice because it's the only thing that passes without giving me nausea. I go to work and eat at lunchtime and when I finish work the stomach ache starts again and often I don't eat in the evening either. I don't know why this happens to me, I sometimes eat fruit, but very rarely vegetables. My diet is not too healthy actually. Unfortunately I also smoke, could it be because of that? What should I drink, eat and or take to be healthier?