Do I have to pay to use the ruku stick?
No.
(Of course if you want to watch Netflix, HBO etc. they must be paid for in some way, but the roku itself and many/most of the channels have no fees.)
But there ARE bogus scam sites you can reach when looking for Roku support or when attempting to link a Roku to a Roku account. They may try to charge you to register your Roku, tell you there's a charge for multiple Rokus, offer some sort of multi-year plan, anything to get you to pay them. And once they have your payment information they can charge your card at will and sell your information to others.