Have an iOS device (iPhone or iPad)? Read about connecting to Apple Health here instead.
Your daily steps should come automatically from the Google Fit app on your phone (assuming you're using an Android device).
If they don't, just follow these steps:
Go to the 'You' screen
Open Settings
Go to Connected Apps
Make sure that the Google Fit is ON
Once this is done, your activity should start being sent to the app automatically.
Using a fitness tracker?
If you're using a wearable tracker that does not sync with Google Fit, read this article to see how you can still connect it to the app.