if you are using Express checkout, there are 3 server behaivors:
1) on the checkout page - Authenticate transaction for Paypal express checkout.
2) on the pp_confirm page - Get Payer Profile for paypal express checkout
3) On the pp_confirm page - Process transaction with paypal express cehckout.
you will need to edit each of these and turn of the the sandbox, you will also need to enter the live server API information.
if you are using a different kind of paypal checkout method (PayPal standard or Direct Paymens) please let me know.