@Sara884,
When you first register your home in Airbnb, the settings for instant booking are already there. Many many new hosts (including myself) have learnt the hard way. I too had to cancelled my very first reservation.
I, as you, were soooo nervous on my next few bookings I couldnt even talk to the guests without having to catch my breath 🙂
Before you cancel, take a close look to the profile of your guest. Read his reviews from other hosts. Some hosts leave OK reviews but dont recommend. He might have three OK reviews but only 1 recommendation. Pay attention to that. Another way to make sure you will feel comfortable with guests is messaging them and evaluating how they respond to you. If you dont feel safe, call Airbnb, explain the situation and they will cancell it for you. Never cancel a confirm reservation, you might get penalised.
Success!!