Which foods that are alkaline can help me prevent getting a stomach ulcer?

My doctor recently told me that I was eating foods that were far too acidic for me. He told me that if I don't lower the acidity in the foods I eat that I could get a stomach ulcer. I'm taking a lot of antacid pills that are supposed to help control the acid levels in my stomach, but I'm also hoping to change my eating habits. So what sort of foods that are alkaline can help me create a good diet to prevent getting a stomach ulcer.