Hi m8, best/quickest way to turn off data is long press power button, press the 'Data Network Mode' then make sure it says 'Data Network Mode Deactivated' Press on it to change it to activated /deactivated.
As for turning off 3G, as far as I am aware you can only turn off data connection on any phone, and not switch between the different data speeds.

 

I think if you change network settings to GSM only, thats the same as 2G. I could be wrong, im used to iphone which does it for me and has a switch which says 3G on/off hehe

 

Well, you can choose between GSM/WCDMA (Auto Mode) or choose either GSM or WCDMA, as MJC said the GSM setting will get your signal ok but will not get your optimum data speed etc, I forgot about that setting, used to use that on my old SE W995 when I didn't have a data plan.
Go to Wireless and Network/Mobile Networks/Network Mode
Edit: ok I have just done this, and where I live I get the fastest data speed I can get for my S2, which is H and or 3g, when I turned it to GSM only, I get the 'E' symbol and also the G, not sure what that means but its wayyyy slower than the usual