You mean use 3g 4g right Im going to assume its 3g as that's the norm unless your lucky enough to live in a 4g area.. Your phone should just connect to 3g automatically when out of wifi range. is your 3g data enabled?

 

After making sure your at least your 3g data is on in settings it should show a 3g in your notification bar when connected. If you have a signal and still no 3g then you may need to do a SCRTN reset (the boost automated system will walk you through it) that will make your phone re-provision its services with the network.

 

that's above my pay grade, but Codegerm is here

 

First I am assuming you are on Stock ROM that the phone came with.

Do you get the 3g notification on your status bar at the top when looking at your home screen?

If you have made sure mobile network data is enabled and your Wifi is off/not connected and you have cell signal and still do not have a 3g connection then a SCRTN reset may be needed. This causes your phone to do the self activation process again. Boost's help menus will walk you through it but basically from your dialer dial ##72786# and answer yes when it asks you if you want to do it. Make sure you are in an area with a cell signal before you do it. Your phone will reset and do the activation process again and query the towers for your network settings again. It does take a few minutes to do this and it will check for updated PRL and firmware also.