Muscle definition is about low body fat. Yes you may lack good pectoral muscle, but it doesn't matter what you have under there if you can't see it. Also you need to focus on strength training more than cardio. It goes Diet>Strength Training>cardio if needed. To much cardio and not enough lifting will just lead to burned up muscle.
So get yourself a routine together and get a good diet going. Read all the stickies in the forum sections of fat loss, weight training and nutrition and its all the info you need to get a great start.