I did this yesterday.
Choosing depends on what your system is and how big a disk it can take (for the BIOS and/or operating system), and what you want to use it for.
All the major manufactures (Maxtor/Western Digital/Seagate etc) have excellent websites, all with software you can download to assist with the set-up. This includes instructions for physically adding the drive. Make you choice and do exactly what it says on the tin. You shouldnt go far wrong.