A clean install is a great thing to do to your mac computer , especially if you have just installed / updated one operating system on top of another , such as a move to Yosemite 

First backup by using Time Machine : Time Machine Guide 

Make sure you can access your backup !

Collect your programs if you need to reinstall them

Make sure you know your Apple ID and password

Erase the hard drive and reinstall the operating system : Format Guide 

Check for updates at the App store ( sign in )

Open Migration Assistant from Utilities and recover your system : Recover Guide

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