Did your doctor say you are borderline or pre-diabetic? Did he give you an A1c test? If so, what was the result? If not, do you test your blood glucose (bg)? If you are concerned about diabetes, you really should get a meter and strips to test so you can see where you are so you will know how to procede. Walmart has a cheap meter and strips, that is what many of us use.I'm not really sure how to "prevent" Diabetes, diet-wise.
If you go to Blood Sugar 101 and read the section on what is normal blood sugar, that should be a big help to you.