Easy.
1. Boot with the Command button+r to boot into Recovery Mode.
2. In Recovery Mode select Disk Utility to reformed the drive and then do a reinstall.
3. Wait for it to finish.
WARNING: Backup before trying to do the clean install!!! Plus get the up your update versions of your Applications from the MacApp Store & MacUpdate.com that are ready for 10.10.
Like any other OS, Yosemite does not like to be started and run on reformed or deformed drives
I'm sure you mean reformat.