If the phone is activated and your current on your plan then go to menu/settings/Wireless and Networks/Mobile Network. Check the box.
You can go to the PlayStore and download a widget app that will allow you to put toggles on any page of your phone you want. Try to get one that has Wifi,Data,GPS,Bluetooth,Airplane Mode, Sound,etc. The more the better.
This will allow you to go to the toggle instead of having to dig into the settings every time you need to change something.
Note: To save the battery turn everything off until needed (Turn Auto Sync on every day for at least 5 min, more often in the day if you have facebook).