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It really depends on what the problem is. A common issue would be the brake light switch , which is on the very top of the brake pedal. The brake light switch prevents the car from being shifted into Drive, without the brake being pushed all the way.
If you follow the brake pedal all the way to the top you should see a little switch that is being pushed by the metal part of the brake pedal. The switch will need to be replaced or adjusted. It is not very involved job it's actually not too big of a task.
Picture of the brake light switch:
Mechanic