I want to connect custom domain into my NationBuilder website. I know I have to pay my domain registrar but not sure about do I still need to pay my hosting provider after I choose nationbuilder host my DNS?
If NationBuilder is managing your domain you do not need another domain host - therefore you can cancel your existing pricing plan with your previous domain host. Please note, there is a difference between domain registrant (where you purchased your domain) and domain host (the company who manages your domain records etc.). Domain registrant is not associated with NationBuilder and allows you to renew/keep hold of your domain name.
Complete and current information on how to use NationBuilder is available in the documentation section.