Apple includes a free Health App on the iPhone that you can use to track your progress towards a goal weight and activities levels.
You can use this app to track steps, stairs walked and more. If you own an iPhone 5s or newer it will track a lot of this information automatically without the need to use another app or a fitness accessory.
You can use the iPhone Health app to connect to many apps and to use the sensors in your iPhone, Apple Watch and other accessories to collect all of your health data in one place on your iPhone.
The iPhone Health app is installed by default with iOS 8 and iOS 9 on the iPhone, and you can use it with a variety of services.