WordPress plugins are obviously quite useful, but some of the best fall under the caching category, as they drastically improve page loads time, and best of all, all of them on WP.org are free and easy to use.
By far our favorite, bar none, is W3 Total Cache. We recommend or use this plugin, it has all of the features you need and is extremely easy to install and use. Simply install and activate, and see your page load faster as elements are cached.
You can find an excellent guide about plugin configuration in the following article: