To install the plugin you need to go to this link here: https://codex.bbpress.org/step-by-step-guide-to-setting-up-a-bbpress-forum/ There you have a detailed description of how to install the plugin & brief description of what the plugin stands for & how it functions.
To get properly arranged with this plugin, you can follow their detailed documentation here: https://codex.bbpress.org/
To set up your forums on your website, please read carefully this part of their documentation: https://codex.bbpress.org/step-by-step-guide-to-setting-up-a-bbpress-forum/#2-set-up-some-test-forums
Note: For the plugin to work properly you need to check this option on: Admin Dashboard / Settings / General / Membership You need to enable the option anyone can register.