moving os x to another hdd

I'm moving my os x to another hdd as I'm getting an hdd and motherboard replacement soon at an apple store, is the time machine back up on external hdd all I need to do? what will be the next steps? Also I think they will keep my hdd, isn't this a risky thing to leave it there since I store much personal data on it? What are my options to avoid any hassle? If you're wondering why I'm doing the back up all by myself and not at the store, it's just because I want to save myself some time.

thanks!

Make a Time Machine backup and also a cloned backup. Carbon Copy Cloner or SuperDuper will produce a clone of your hard drive. Once you've done that and verified the backup, erase your internal hard drive on your Mac before taking it to the Apple store. Then you won't have to worry about your personal data being compromised.

You can erase the drive as chscag suggests using the Snow Leopard or earlier DVD ~ boot from it holding down 'C' immediate;ly after the chime, going into utilities > Disk Utility > Erase. For Lion or Mountain Lion use Command+R.

http://support.apple.com/kb/HT4718