Updating software question

[tl01]

May this be the last question I have for the week!

I have an older iMac that I want to upgrade to Yosemite. I have to clean off the hard drive too. I have a drive I will use for time machine back ups and anything else necessary. If I want to start fresh and upgrade to Yosemite... Is there a way to do this using the hard drive? If so, how should I partition the drive first? Can one download the OS to the back up drive?

Or is there a better way to do this?
Thanks!

 

[Samuelsan2001]

Use an 8GB thumb drive to create a bootable usb of yosemite.

Here are the instructions

https://support.apple.com/en-us/HT201372

Then you can reinstall any apps and migrate your data from your time machine back up.