The easiest way would be to use an in-built or plugin cache facility if you are using a CMS that has this. Secondly, you can use a web server module such as apache mod_cache if your hosting allows installation or modification of the web server. There are also applications like varnish or squid that can act as a cache in front of apache, however setting this up requires a bit more knowledge of server administration.
It's quite easy to write caching code. On each page load, check whether a cache exists and is within an allowed 'age', if so open and serve this file, if not go through your normal page generation and write it to disk and serve.