You'll also have to enter your height and weight to begin using Google Fit, as that way you can know if you're losing weight. Of course, even if you enter all your exercise plans on Google Fit, if you don't actually do them the app will remind you and you'll see their absence reflected in your graphic. Thus you'll have an extra incentive to exercise.
Google Fit is a very useful app, like almost every other one developed by Google, and can also be connected to your Android Wear device or even consulted on its official website.
Requires Android 4.0 or higher