Your best bet will be looking at the WifiManager class. Specifically the setWifiEnabled(bool)
function.
See the documentation at: http://developer.android.com/reference/android/net/wifi/WifiManager.html#setWifiEnabled(boolean)
A tutorial on how to use it (including what permissions you need) can be found here: http://www.tutorialforandroid.com/2009/10/turn-off-turn-on-wifi-in-android-using.html