Used mac mini, how to start fresh?

I've had a iPhone 6 for a while, but this is my first mac computer and my first ever apple purchase. I'm very excited to be on this device. It's a late 09 mac mini with the 2.26ghz ghz and 2x2gb ram and a 320~gb drive.

I've been playing with it for about a half hour, I just got it in the mail. I bought it used but it only came with the mac mini, power supply, and a remote. How can I reinstall mac os? Will I need to buy it? I don't have any disks unless I can make one. El captain is the latest version my mac supports. If the mac keeps up to my expectations, I want to buy a ssd for it.

I'm pretty good with windows and linux, built gaming computers and home servers, but have just about 0 experience with mac.

How can I reinstall mac os, in my situation? ...Just found out the "About this mac" says it's a early 2009. Not sure if this is good for me.

Thanks,

Ben_Kenny