SIM card for iPad 2 3G question?

SIM card for iPad 2 3G question?

I’ve recently bought an iPad2, an I wanna get a sim card with O2, does the actual sim card cost anything, if so how much, and does my rolling contact start as soon as I buy it? Or do I have do activate the sim card via the Internet/phone, I wanna know because I don’t need a sim card for it yet, but if I ever needed to I could just slot it in whenever I need it.