How to turn my iphone 4 3G off?

I'm going on holiday and need to turn everything off so I don't waste money.
How do I turn off 3G on my iphone 4?

Well the simpler thing to do is enable Airplane Mode. I think you should know where that is.

Settings -> General -> Network -> Enable 3G (also turn of cellular data if you turn of 3G)

You can use airplane mode which is right at the top of the settings. Turning this on means it won't interfere with the radio transmitter on the plane but you can still play games or use apps. You won't receive any messages or 3G/ Wi-Fi until you turn it back off again, and won't be able to text anyone while on holiday either, therefore costing you no money but being able to keep your iPhone on. Hope you have a good holiday.

Either turn on airplane mode, (Settings --> Airplane Mode --> On) this will enable you to keep your phone on but will turn off all 3G so it won't be able to download emails, texts or any push messages.

Or

Just turn off your phone & keep it off for the whole trip. (Hold down the button on the top right of the handset until a swipe bar appears on the screen, then swipe to turn off).

Hope this helps.

Scott's answer means you won't be able to send or receive calls or texts. If you are wanting to turn off your 3G to save battery and cut out data use, but still mean you can make calls & send texts, then go to:
Settings
>>General
>>>>Mobile Data

Then turn off Enable 3G. When you come back home, remember to switch that on again)
Also turn of Data Roaming - this can stay switched off all the time, you don't need it switched on when you're in your home country.

Settings - Cellular - "Cellular Data OFF"
"Turn off cellular data to restrict all data to Wi-Fi, including email, web browsing, and push notifications."
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Meaning you turn off your 3G and other related connectivity unless you're in a hotspot or at home on a Wi-Fi Router.