Different iPhones require different sized SIM cards.
For an iPhone 3, 3gs or 4 you need a standard SIM, for a 4s a Micro SIM and for an iPhone 5 a Nano SIM.
Once you join giffgaff you'll get:
Woohoo. Get 500 UK minutes for only £10 a month. Plus unlimited texts and 1GB mobile internet.
More info on prices