Welcome to the Community!
That's a very good question! Firstly, you should note that your billing cycle usually starts a day or so after you activated your line. However, people are generally able to use their services immediately after activation. Any usage prior to your actual billing cycle start date would be prorated for the day or few days before the cycle started. Those prorated days would likely be the previous balance you note. You can view more information here.
You should also note that for every billing cycle, you will be charged for the following month's usage as well as any additional usage (ie long-distance, overages, etc) from the previous month.
Hope this helps 😀