First of all, if you have never used linux and you have precious little free space, take a look Knoppix. It is a linux that doesn;'t need to be installed and runs straight from CD.
the Mandrake installation will ask which partition you want to install to and it will format that partition so make sur eyou choose the right one.
Once it is all installed, you'll get a boot loader which detects other operating systems on the pc and lets you choose which to booth to. You can edit this to set a time out and the default boot option so you can boot then go and make a cup of tea.