When NFC is turned on, can you pay for something at a store, or do you need to install a special chip?
If you have an NFC chip (and a Secure Element (SE), but as far as I know all current NFC phones have both) then theoretically you should be able to pay for goods using Google Wallet, however only certain phones on certain networks are supported and it can only be used in the USA.
EDIT: Just to make it clear it may still be possible to pay for goods via NFC using your phone without a SE but if you want to use Google Wallet then you'll need both.
It's feasible that companies could have their own custom payment application where you load credit onto it and have an ID/cryptographic key stored in your device and then pay using that. I don't think this is too likely however because of the security implications of someone else using your ID/key and also, what would the benefit for the merchant be? They now have to support this NFC payment infrastructure and it saves them only a small percentage of money on credit card fees.
Any phone that supports NFC has the necessary "chip" already (that is, the hardware supports that kind of transaction). The crucial factor is the software running on the phone. Unlike with magnetic-stripe and chip-and-PIN credit card transactions, there's no one standard: there are several competing contactless payment systems. Google has Google Wallet (but only in some countries), Paypal has one, Orange (a phone network) runs one in the UK. In addition, most Bitcoin clients (such as Bitcoin Wallet) can use NFC to exchange payment details, but you still need a network connection to actually carry out the transaction.
If you sign up for one of these payment systems, and you install their app on your NFC-capable phone, then you can use your phone to pay at any shop that supports that payment system. There's no extra hardware for you to add to the phone.