The first step is to populate payment methods for your members to choose from. You can add manual payment methods by going to Setup » Automated Payments » Add Payment Method » Add Manual Payment Methods.
You can add automated payment methods, such as credit cards or direct debit by going to Setup » Automated Payments » Add Payment Method and choosing from the list of existing integrations. You can read more on what options are available in our 'Credit Cards & Direct Debit' guide.
You can now enable payment requirement features by going to Customize » Features » Payments. To make it so all members are required to add a payment method, click on 'Activate' next to "All members must set up payment method."
Once this setting is activated, new members won't be able to do anything within their member portal until they have added a payment method.