G'day and welcome to the forums.
Tell us about your 'outer. Unless you have at least 8GB of memory, Yosemite may not be for you. Assuming you do however, first off backup to an external hard drive. Personally prefer cloning software to Time Machine. DO you have Yosemite on a bootable USB thumb drive?
If not you need to download it again. Don't run. Use Diskmaker X to make that bootable 8GB thumb drive. Thne reboot, with the thumb drive attached, hold down 'C' to access the Installer.
Leave Installer, to go Utilities > Disk Utility and erase the drive and format Mac OS Extended (Journaled) and under Partition > Options ensure GUID is selected. Back to Installer and let it run.
Hook up the backed up external drive, go to Utilities > Migration Assistant to copy everything over. Be aware if your problems have been software related, you may migrate them also. How large is the hard drive and how much free space after migrating?