How long will it take for me to become anemic?

I have turned vegan this past month. I only eat Bananas, apples, gs, melons, bell peppers, lettuce, Tuna fish, & ocionally a granola bar. I don't eat aton of food. Im not a big eater. How long will it take for my body to lack certain nutrients & become ill? im really concerned. What vitamins do I need to stop becoming anemic? thanks.