how do I turn off my sim card to use just the wifi?

You can turn cellular data off at Settings > Celluar Data.

If you keep Cellular Data and wi-fi on, your iPad should connect to an available wi-fi network automatically that you have access to and have connected your iPad with before when returning within range of such a network.

im guessing u have a 3g ipad 2 all u have to do is turn on airplane mode in settings the this will disable 3g and the sim card and wifi to put wifi back on. go in settings and tap on wifi and tap (on) then it will be enabled. hope this help. and 3g should be disable and sim card

When wi if is available it will use wi if 1st. If not, then 3G.