I have qualified NDA 2 2019 written exam,what to do next for SSB? Is there any registration process for it?

Yes after clearing written an email will be sent to you. You have to select dates if given option or else they will send you your concerned SSB centre where you will have to attend and on which date. If you will have to check the email. In email they will mention all the details. Email will be sent on registered email id which you have given during registration. You must even see at the result section where they will mention important details.