Do you want to keep a Windows partition on the disk or not?
Either way, put the Mandrake 9.1 CD in your CD-ROM drive and reboot. The installation will start.
The install can resize partitions for you, and takes care of everything.
If you want to obliterate your windows installation (and are not happy repartitioning), then you can install the OS on the first partition (/dev/hda1), and allow /home to be mounted on the second (/dev/hda2), giving you 20GB for the OS (way too big IMHO), and 20Gb for /home. The installation procedure will also let you remove the existing partitions, and create one 40GB partition in its place, or four 10GB partitions, or whatever you want (especially with an Extended partition)!
You are asking a very broad question which as been answered many times before both here and elsewhere. Try
www.tldp.org and
www.google.com for more.
Cheers
ZB