A WordPress Plugin is a program, or a set of one or more functions, written in the PHP scripting language, that adds a specific set of features or services to the WordPress weblog, which can be seamlessly integrated with the weblog using access points and methods provided by the WordPress Plugin Application Program Interface (API).
The first thing to do is to search various WordPress Plugin repositories and sources to see if someone has already created a WordPress Plugin that suits your needs. A Google search will bring back a plethera of both free and purchasable plugins. Be sure that the plugin is compatible with your installed version of WordPress.
WordPress Plugins are designed by volunteers and are commonly free to the public. Plugins listed in the sources below have usually been thoroughly tested and considered "safe". But remember, Plugins are the responsibility of the author and the user, and they are typically works-in-progress as WordPress grows and expands.