The iPhone 5 and above require a nano SIM which is smaller than all previous SIM cards.
Existing customers who purchase an iPhone 5 outright can come in store and have their SIM card re-burnt free of charge. But remember, if you buy an iPhone from another mobile carrier or one of their dealers, it may be locked to their network, so make sure to check for any unlocking fees.
Under no circumstance should you cut a micro SIM card or Next G SIM Cards into a nano SIM using a SIM card cutter. This is a non-approved process and voids the warranty of your handset.