normally, you use the dos command, FORMAT (search your C: drive for FORMAT.COM)
but, it used to be, if you don't partition it first, you would lose everything already on it. with the latest versions, I don't know.
Partitioning is dividing up one hard drive into two or more hard drives, which are the addressed separately.
For example, you probably have an A: drive - the floppy, a C: drive, you hard drive, and then the D: drive, the CD (or something similar. Partitioning your C: drive, say 40GB, into 2ea 20GB drives, and your C drive becomes a 20GB, your D: drive is a 20GB, your CD becomes E: drive, etc.
I always partition my main hard drive, and put all my data on my D: drive (that way, if I screw up my C: drive, my data is not gone - unless there is a real physical problem with my hard drive), and also, when I screw up I can re-format my C: drive, reinstall all my applications, and still have my data, safe.
the best bet is to buy a program called Partition Magic: