You may review the available payment methods in Magento via the admin panel by clicking on the Stores menu > Settings section > click on Configuration.
On the next page, expand the Sales menu on the left and select Payment methods.
This way you may review all of the available payment methods on the newly opened page.
For the purpose of this tutorial we will enable the Braintree method which would allow you to accept credit/debit cards and PayPal in your Magento store. You may enable it by clicking on the Configure button next to it.
Enter the method's title, setup the environment (Sandbox or Production), define from which countries to accept payments, the range of the accepted payments and other options you want to alter.
In the Sort order field you should enter the position of this payment method compared to the other payment methods offered to the customers.
That's it, you have just enabled this payment method for your customers. Since all payment methods have different configurations, we cannot explain in details how to configure each one of them. Simply follow the instructions on their specific pages and they should work.