Mobile internet is great when you're out and about, you can use it on your smartphone to load web pages, use map apps, upload photos or just check Facebook. Naturally it's a pain if your device has no 3G connection. See also: How to manually upgrade an Android smartphone or tablet.
This could just be a problem with your network so check the live service status before you go changing any settings. If you've moved your SIM-card to a new device then you might be sent the settings for internet but this doesn't always happen or it simply fails.
Whatever the reason is for your lack of data connection, here's a step by step guide to manually setting it up yourself.
Set up mobile internet on Android
Navigate to the settings menu of your Android smartphone. This can be done via an icon in the apps menu on any Android device and sometime via the notifications bar.
In the settings menu, you need to find the wireless and network settings. On most devices, click the 'More' button but this might not be same for your specific device.
One you have found the wireless and network settings, click on the Mobile Networks button.
Make sure the mobile data option is ticked. This might solve your problem but if not, click on Access Point Names (APN).
You might already have a list of different APNs to choose from so you might be able to fix the problem by simply selecting the right one. If this doesn't work then click the menu button in the corner and select New APN.
Here you need to fill out all the details for your particular network (you'll need to contact your network operator for these details or try a Google search for them). Enter the details and click Save in the menu.
Your new APN will now show up in the list for you to select. Give it a few moments to connect and you should have your internet connection. If you still don't have a connection, check your APN details have been correctly entered.