Modify and activate Stripe payment gateway on existing WordPress site

I have an existing WordPress training website with LearnDash LMS plugin to run quizzes and tests. It has Stripe payment gateway integrated already but the site was offered free for users for a year. Now I plan to make it a paid site and wish to activate the payment gateway, make one quiz and one test free, but the rest of the features can be accessed only by registered members who will pay a monthly fee. This is the first step to be completed.
On completion, I will have further updates and changes to be added to the site.