Do I need to back up my iMac to an external HD? if so should I partion the drive and use it for storage as well

Question

At a loss... 

Answer 1

Highly recommended you have at lesat the incremental backups automated by Time Machine (the software).  Time Machine makes backups so easy to setup and after that you ‘forget’ about backups as all tasks occur in the background, hourly by default.
If you use Time Capsule (the hardware) for your backups, you should not partition that drive.  If you use a conventional external drive, you can consider partitioning it so that one volume is dedicated to Time Machine and one volume for example for other data purposes.  When you have an external drive, you need to drag it to the Trash icon on the dock to eject the drive before you shutdown (or restart) the computer; this will ensure all data is physically written to the drive and buffers cleared.  When you partition the external drive, ensure that both partitions are ejected before you shutdown.
The first time Time Machine runs, it does a full backup and can take quite a long time depending upon how much you are backing up.  After that, it only backs up data that has changed and is usually very quick.  At anytime, you can delete those backups and do a new full backup if you wish.  It keeps old versions of everything you backup as long as you have space on the drive.  When you are nearly out of space, Time Machine deletes the oldest versions of files until it has made enough room to continue backups.  You’ll always have at least one complete image of all your data backed up assuming your backup volume is always larger than the data you wish to backup.  If your backup disk is not large enough to hold everything, you can select files you do not wish to be backed up by Time Machine, for instance, you may have large video capture files from HDV tape that you do not need to backup because they are already on tape.