how do I restore my mac mini and back up selected files

Hi everyone,

I have a question as I'm a little uncertain how to handle this:

I have a Mac Mini 2011, Processor 2 GHz Intel Core i7, Memory 4 GB 1333 MHz DDR3, Software OS X 10.9.4 (13E28), with SATA Disks of 500GB each. I have noticed it remarkably slowing down lately and I when I checked storage, it says I have 8.65 GB Audio, 117.21 GB Movies, 24.09 GB Photos, 17.34 GB Apps and 269.84 GB Other!!! I guess my first question is what is that 269.84, but I still would like to back up the things important to me (like photos and misc work files) and just restore my mac to factory settings. Is there any way I can send my Photos and files to my second SATA disk and restore the start-up disk? (please explain as though I'm new to this... 🙂

Thanks in advance!

Mike