The
Venn Diagram method.
Understand the nutritive/caloric properties of food you eat. Use this knowledge to tell yourself rationally which foods should not be eaten, which should be eaten in moderation, and which should be eaten liberally. Understand the health consquences of eating too much pizza, cheeseburgers, red meat etc.
Find out which foods you like, which foods are healthy, and foods you have yet to try which are healthy, draw up a Venn Diagram. Eat the intersecting area. Continue to tweak your eating habits as your personal tastes and health needs progress.
Exercise: Find out the types of exercise you like. Find out how much exercise you require to meet your goal. Find exercises you are curious about (many people would love gardening if they tried it etc.) Draw a Venn Diagram and do the exercises you like. When you notice you have not been exercising you have failed at doing things you enjoy. Making things routine will help a lot and lift your mood, you get free endorphins everyday (high on life?), you are getting in progressively better shape, and you are having fun.