Practice installing it first.
Install Debian as a guest OS in a virtual machine.
Play with it and mess around a bit.
Once you know exactly how to go and install it to a real hard drive parition, do so.
If you're not sure or confident, then you need to mess around more.
Read : Debian-Reference, Debian-Handbook, Debian Newbie's Guide, Debian-Wiki and anything else Linux or Debian related.