Your first bill will be slightly higher than other bills as you are paying for more than one month of usage. This is because your first bill covers the period from when your service is connected up until your first bill, as well as your price plan for the month ahead.
For example, if you had your installation appointment on the 24th of November and your first bill issues on the 5th of December, your bill will include a charge from the 24th of November to the 4th of December called part-period charges and credits.
You would then pay a charge for your price plan for the month ahead (from the 5th onwards).