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.
WordPress Plugins are available from several sources. The "official" source for WordPress Plugins: Official WordPress Plugins Repository
Not all WordPress Plugins make it into the above repositories. Try searching the web for "WordPress Plugin" and the keywords for the type of functionality you are seeking.
How WordPress Plugins Work
WordPress is conceptually simple and PHP is pretty straightforward (it’s a scripting language). SQL is SQL, arcane as always, but totally standardized.
Put them all together, though, and it’s somewhat dizzying, at least for a newbie at it like me. The structure of WordPress plugins is regimented, but is too complex to allow one to just dip a toe in the water. Plugins may require modifying WordPress files and templates.
Familiarity with PHP, HTML, CSS, and CHMOD may be necessary.
Modifications may be necessary if the color scheme, font styles or some other reason will make the plugin mesh better with your theme. Often the plugins offer options in the management module in WordPress’ back office but just not quite the tweak option your seeking.
When this happens you will find yourself exploring the plugin editor and delving into the .php and .css files. This is part of the fun of site development and WordPress life, just remember to backup your site before starting and you should have hours of fun.