Most people have learned through experience that white sugar and refined flour are inimical to good health, and they know how difficult it is to give these things up in a society whose eating habits are based on them. It is relatively easy to replace margerine with butter and refined polyunsaturates with extra virgin olive oil because these fats taste so much better; but sugar and white flour, being mildly to severely addictive, are harder to renounce.
First, just start replacing white flour products with a variety of properly prepared whole grains and limiting sweets to occasional desserts made from natural sweetners (such as honey, pure maple syrup, rapadura, date sugar, molasses, etc.).
It may take time, and will almost certainly have setbacks (as I did for several months), but in the end your willpower and persistence will reward you with greatly improved health and stamina. Not only will you feel great, but you'll loose pounds and look great!