If you want to use MySQL with Apache, you could try to download components and modify the default Apache installation manually, but I find it much easier to download and install an entire separate server suite. While there are many options out there, I recommend MAMP.
Extract MAMP, double-click on the .dmg image, and drag the MAMP folder icon to the Applications folder. Once it is complete, look inside Applications for the MAMP folder that will have been created for you.
Start the MAMP application. This will start up your default browser to show you that the new server is working, at the following URL:
I strongly recommend that you change the MAMP application preferences. Set the Apache Document root to the folder that contains your site. (When you set the Document Root to a new location, the Apache server will stop and restart, which will take several seconds). The URL to visit the locally hosted site served by MAMP will be: