I always like to have a CDR burnt/handy of Gparted. http://gparted.sourceforge.net/download.php
I stick that in my DVD Drive and boot directly from the CDR before I install any Distro. Then I just select manual when installing my Distro. Gparted will Display the current Partitions and easily allow you to do what is needed before you do the actual install. You may want to give it a try to see what it tells you.
Typically your first Partition will be "/" with something in the order of 15-30 Gig. The remainder of
Hard Drive space, Minus 2 times RAM, will be "/home/user", and the remaining 2*RAM is your "Swap".
But, this can vary as you require. You may make the first partition just one complete space, or
use one Partition with Swap which is typical 2*RAM.
REMEMBER there is a MAXIMUM of 4 Primary Partitions on any one Physical Drive. But, you can also have
Logical partitions with several partitions inside the Logical Partition.
If you are setting up a dual boot, then be careful to not destroy the OS that boots first, along with it's MBR.
Most Motherboards/BIOS now days will boot from the DVD/CDR Drive or USB, in the Order they are selected in