What do you plan on using your system for, (games, video editing, graphics, business,...etc)? Anyone here could give you some good advice if we knew your purpose. Partitions are a good thing for organization, saving space, business, pleasure..., but if you don't have partitions set up already, and you have data stored on you HDD's you better use a program like Partition Magic to safely create them. Otherwise, you will lose all your data.
However, if you are first time setting up your system, use good judgement and planning to establish the size you will need for whatever you plan on doing.
Be more specific and someone here will point you in the right direction.
JC-------<*){{{>{~~~~~
Fisher of men