You ...
... start up "Disk Utility" and choose the harddisc you want to partition; do then select the according panel in "Disk utility "(located in the right upper area).
Choose your first partition according to your system needs; as you mentionned 30GB usage at the moment, I would recommend nothing huge, around 120 to 250GB. If you have a spare drive of this size lying around, use it as external backup for a cloned copy of your system drive. If you haven´t considered this yet, now is the time to get an external drive for backup.
Now, don´t use the second partition for anything which needs harddrive access while you are working with your machine - it will slow down your system drive severely, when the read/write head jumps around those partitions. Do only put stuff on it like video files your apple tv might want to look at, document/file drawer, software installations, asf.
My setup, e.g.: a 500GB drive partitionned into 200GB (system) and 300GB (all my software and system installation packages). My system drive is time machined on another one, but also replicated (with "Carbon Copy Cloner") to an external 200GB harddisc; and on a third hd there is a DMG image of it. I keep all of my data sets (mostly dv video and aiff recordings) on other drives. Keeping heavily used data files of the system drive will speed up your workflow quite significantly. Same applies to cache files, like Final Cut Pro or Photoshop are using.