How can a chocoholic learn to stay away from chocolate to lose weight?

Reduce the amount you eat each day, until you get down to only one piece a day. Do not eliminate chocolate from your diet completely. That will make you yearn for it even more. We usually want what we cannot have.

Instead, have a small piece of dark chocolate every day. Dark chocolate in a small amount is good for you and your health.

If you know that you can have that one piece, you will not feel deprived, and you will decide when you want to enjoy it.

If you feel you must have something sweet after a meal, brush your teeth instead. Brushing is good for your teeth, and will take away the urge for sweets.

Substituting another type of sweet for chocolate is not going to get you to break the habit. It will merely exchange something else for the chocolate. One good way to cut down is dont buy any, dont have any in your home that way if you get tempted its not there.

by asking them do to join an exercise program

Eat chocolate in moderation. Plan a exercise goal and once you acheive that goal, reward yourself with a small piece of chocolate. Chocolate is actually good for you and increases the release of seratonin( the body's feel good chemical).

ADD ON:

There are types of chocolate out there that are dark and made mostly from cocoa extract. The ones that are good healthy chocolate or those that have between 90-100% cocoa extract. This actually helps with blood pressure and to regulate sugar levels.

ANOTHER ADD-ON:

Find many healthy low-calorie foods that you like better (it takes time to acquire the taste for these), and little-by-little, replace bad habits with healthy ones. Do it not merely to lose weight, but to gain health, feel better, and live longer. It saves money, too.
However, if you diet for the reason of gaining self-esteem, you will get tense and have trouble (esp. if unsuccessful). If you diet for reasons like health, you can relax, and it will be easy. Enjoy the process-- being healthy is the greatest feeling in the world.