If you've recently activated your phone and it's not yet able to make phone calls or use the Internet, performing a Profile Update will force the phone to request its device configuration from the network which should get you up and running.
There is some small variation between Android phones as to where the Profile Update command can be located, however, you'll generally find it in one of a few places in the Settings menu:
On your phone:
Settings > About phone > System Updates > Update Profile
Settings > About Phone > Update Profile
Settings > More > Mobile Networks > Carrier Settings > Update Profile
Once you've selected "Update Profile," all you need to do is wait until the update completes and then reboot your phone.
If you have a problem completing the update, make sure you're in an area with good, strong signal; then disable Wi-Fi and enable data.