Many payment options but one can be used by the customer

I need to enable "Cash on delivery" and "Paypal" payment method. When the customer submit order he can only choose "Cash on delivery" (I just want the customer submit order and do not make payment). After Admin review the order and choose "Paypal" and update the order, the customer can use Paypal to checkout.

Please help me how to make settings or modifications.

Thanks a lot.

Check the geo zones?
Also which Paypal extension are you using?