Q: My iphone is ex contract can I put a pay as u go sim in it?

If supported, your carrier has to start the unlocking process, check this article about it:

iPhone: About unlocking

If it is o longer under contract AND has been unlocked by the carrier, then yes if it is valid sim.  But, the iPhone must still be unlocked by the carrier first.

If the iPhone is carrier locked you need to contact the carrier and ask them to unlock the iPhone.  Only the carrier can unlock it, Apple does not get involved and user cannot do so themselves.