If you have a fairly decent smartphone then yes you can share your data connection as a wireless access point. If you're on android, you can check if your phone has this capability by looking under the settings/wireless and network and seeing if the option is there.
However, this is not always counted as part of your standard data charges/allowance, it's best to check with your network provider about this. Also it tends to kill the phone battery, so it might be worth getting a back up battery.
Other than that you can get a wireless mobile/dongle thing, which runs on batteries and creates a little wireless network for itself which you can connect an iPad to. You'll need to put a sim card into this, not a normal phone sim, but a data only sim, and the charges are the same as the USB dongles you can get for laptops/computers (ie, pay monthly or pay as you go, although with PAYG sometimes there's a time limit as to when you have to use the data by)
If you get the 3G iPad it just has a SIM card holder, like a phone, which you put the data sim into (it should come supplied with one) and again the charges are pay as you go or monthly, but the iPad will be able to connect via wi-fi as well so you won't necessarily need to use it all the time, you could save data-heavy actions like updating or downloading apps for when you're at home.
Hope that helps a bit? TBH if it was me and I wanted it mainly for using out and about then I'd go for a 3G one and save the hassle of having an extra thing for the batteries to die on.