If you are charging for your memberships, you need a payment processor (payment gateway). The two most popular gateways are STRIPE and PAYPAL. Paypal has a number of different gateways so its important to configure the correct one if you do use paypal.
If you’ve been using the internet for any length of time, you’re probably aware that SSL is the basic security component that underpins a lot of your interaction with sensitive websites such as banking and payments. In order to accept credit cards on your site, your site MUST be set up with an SSL certificate and serving your cart and checkout page using that SSL certificate.
An SSL certificate is a “signature” that allows visitors to verify that your site is exactly the site that they intended to visit. And it allows for encrypted communication between your WordPress server and their browser – which makes it hard for hackers to snoop on the connection and steal credentials and credit card information.
Stripe, Paypal and other payment providers and gateways will simply not allow you to collect payment information and send it to them without SSL.
It is best practice these days to serve all website pages over SSL – not just the checkout pages.
Your host or WordPress admin is generally responsible for securing and installing an SSL certificate. A few years ago this process was a PITA. But these days most hosts allow this to happen with a couple of clicks for most users.