BuddyPress.org

1. Before installing any plugin – BP or WP – double-check the compatibility of the plugin with the version of BP/WP you have installed. If the compatibility is not there and you choose to install the plugin, be prepared for errors and to tackle it. In other words, the safe bet for a novice is not to install plugins which are not compatible with your installation.

2. One way is via https://wordpress.org/extend/plugins/s2member/ I recommend reading through the documentation http://www.primothemes.com/post/product/s2member-membership-plugin-with-paypal/

3. Something like this – https://wordpress.org/extend/plugins/theme-my-login/ and redirect visitors who are not logged in to the login page