The Workout app on the Apple Watch allows you to set specific exercise goals while tracking your progress and keeping a summery of your results on your Activity app. Here are a few basic tips on how to use the Workout app. You can read more about the Activity app by hitting the link above.
To get started, tap the Workout app icon on your Watch then select the workout type you want to begin. This will take you to the "goal screen" where you can set your calorie, time, or distance goals or no goals at all. Just swipe left and right and use the Digital Crown to set goals then hit Start to begin your workout.