You tell them how you are going to solve a problem for them, or how you are going to prevent them from having a problem in the first place. You need to use these in your headlines too. Spell out specific benefits in both your headlines and in the body of your ad. Talk directly to your customer in your ad, telling him what you are going to do for him. Many ads don't even tell the customer WHAT they are selling.
Many online people are sooo fed up with trying different hyped up opportunities.
After you have a good headline and good body copy you need a call to action. You need to tell the customer what action you want him to take. Tell the customer what he should do next. Your ad may have him marginally interested but he often will not take action unless you tell him to. Give the customer several ways of taking advantage of your great offer if appropriate.