Fist thing here is you'll need to identify who's HD your system has as you'll need to replace it with the same vendors drive (i.e. WD for a WD or a Seagate for a Seagate). If you want to use a different vendors drive you'll need to get the correct thermal sensor cable for your new drive.
Cable, Temp sensor, Hard Drive:
- 922-9215 - Hitachi drive
- 922-9216 - Seagate drive
- 922-9217 - Western Digital drive
The HD you get needs to run at SATA II (3.0 Gb/s). You'll need to check the spec sheet of the HD you plan to get to make sure it will work. As an example here is a Seagate SSHD drive speck sheet: Seagate Desktop SSHD spec sheet. Here you'll note the line: SATA Transfer Rates Supported (Gb/s) and it offers 6.0/3.0/1.5 Which is SATA III, II & I all three supported (auto sensing).
Here's the IFIXIT guide for your system: iMac Intel 21.5" EMC 2308 Hard Drive Replacement to help you out.
OK, now what?:
Well you'll need to find a friend who has a Mac as you'll need to setup a USB thumb flash drive as a bootable disk and then copy over to it the OS installer so you can prep and install a fesh copy of OS. Your other option is to get a SATA to USB adapter cable and then setup your drive directly off of your friends system. Be careful here as you'll need to get an adapter which has an external power supply for this to work.