To start recording your mileage, you can press the green "+" button on the home screen in the app, then select "Track miles." Once you're finished tracking any business-related mileage, you'll tap on the blue "I'm done driving button."
Not sure which miles to record? If you're a rideshare driver, here's where you can read a bit more about which miles are deductible. If you're not a rideshare, here's where you can read about your deductible mileage.