To activate and adjust mobile networks on Android phones and tablets, go to Settings > Wireless and Networks > Mobile Networks.
Note that on some Android devices powered by Android 4.0+ (ICS and Jelly Bean), such as Samsung Galaxy Series, Google Nexus Series, you need to click on More Settings after going to Setting Menu
By enabling Use Packet Data (or Mobile Data), you will be connected to internet.
The next option is to connect to data services (Internet) when Roaming is active. You can adjust Internet settings via Access Point Names, like the Port, APN, Username or password (if required), etc. Network Mode lets you choose which networking mode you need: 3G, 4G, or 2G. The last option will let you select a network operator by searching the area for any operators.