Disk Utility

This is my first time ever of doing a "format" & "Reinstall" of an OS. I long ago upgraded the internal HD IN MY 2009(late) Macbook.

AM going to need to know Which volumes of my hard drive and/or anywhere else in it that I need to erase prior to reloading Yosemite (and any other predecessor OS') can anyone provide my a link to full instructions on How to Used Disk Utility? I have All the data I need (photos, music & Apps) already saved to an external HD.