If you already have an Internet data plan, you should be able to connect to the Internet after you activated the service. This can be done from the Settings menu of your device. After the service is active, open your browser and you can start surfing the Internet. If you can't do this, it's possible that you must do some settings that are related to your provider. They can be different depending on the provider, so it's advised to contact Customer Support in order to fix your problem.
