Choose a sturdy log. Start pecking. Don't give up before you feel confident.

Dips, sit-ups, curls with dumbells, push-ups, Benchpress, military press, declines and pull ups. (NOTE: Always lift wieghts with a partner)

Seriously, I would say that exercises such as the bench press are the best at first. These are compound exercises that help to buildup your chest, shoulders and arms together, and generally compound exercises help to stimulate the most growth because the exercise uses much more muscle than isolated muscle exercises. If you use a large group of muscles, you will cause more microtrauma to the muslce fibres, thus stimulating increased muscle growth and repair. Though this sounds dangerous, this is how muscles grow naturally, and is ocurring to you every single day anyway. The more trauma caused, the more hormones and so on will be released to stimulate muscle growth. So this is to increase your bulk. To increase the definition and tone of the muscles, work with "pec decks", the machines that you sit at, grasp a bar on either sides of you and bring them together at your front. Similar machines that work in different ways also exist, as a gym trainer to show you. These work the pecs in more isolation that the bench press (though you will still use your arms). Press ups, and forward facing tricep dips with your body leaning forward also are good exercises for your pecs. Obviously you should also have a good cardio workout to reduce the fat content, and improve the definition.