If you're running Linux, you know it might be a bit different on your device, but here's a great walk through from one of our users if you're using Mobian.
If your device doesn't support configuring the APN automatically, you can manually enter your settings by following the bullet points:
Open the Settings.
Open Mobile settings.
Under Advanced, open Access Point Names.
Press the + at the top-right to add a new access point.
Below are the fields which will have to be filled in with the information provided:
Username: *leave blank*
Password: *leave blank*
Press "Save" at the top-right.