"That sucks. I really wanted to back up my mac. Now i can't because the stuff on my hard drive is just too important to erase. What's HSF+?"
HFS+ is the Mac file system. I'm going to -guess- that the external drive you have is formatted in something else compatible for use on both a Mac -and- Windows. That won't do for TM.
Suggestions:
If you really want to use TM, you'll need -another- external drive. But what I'd really suggest is one of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=usb+sata+dock&x=0&y=0
(many items shown, they all work the same, just pick one you like. Some can be had for as cheap as $25. You might consider getting one that's "USB3", will be faster than USB2 with the Macs of the future)
Then, just get a "bare" hard drive of your choice to go into it.
A "docked drive" will behave just as an external drive.
BUT --
The -best- Mac backup solution doesn't use Time Machine. Instead, download the free "CarbonCopyCloner" app from:
http://bombich.com
It's easy to learn and use and one of the best pieces of Mac software out there.
CCC will create a "bootable clone" of your internal drive (you CAN'T boot from a TM backup in a moment of extreme need). The files on it are in POFF (plain ol' finder format) -- just connect the backup, and it will show up on the desktop, ready to use.
CCC will also do "incremental" backups so you can maintain the clone in a state that is exactly like your internal (at the time of the last backup).
I have seen post after post from folks here who used Time Machine to backup, and then -- in a moment of extreme need -- find that they can't access their TM backup to get going again. Use CCC, and this won't be the case.