Is the device brand new? If it is, there will be system updates waiting to be installed as the device has received updates since it was shipped from the factory to the retailer. These updates are recommended to do and you can only do one of them at the time and once you have installed one and restarted the device, you will receive a new notification about an available system update.
The updates vary in size and if you have a limited data plan, it's recommended to update the device when you are connected to a Wi-Fi network, or if you have access to a computer, you can update the device using the Xperia Companion software that you can find on the link below.
Until you have access to a Wi-Fi network or can update it using a computer, turn off automatic updates in Settings > (System) > About phone > Software update > tap the 3 dots in the top right corner > Settings > Auto download system updates > Wi-Fi only.