First of all, I'm just going to assume that you're using Mac OS X only, and if you're not, just ignore my post
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The first step in repartitioning your drive is backing up all your data that you want, but seeing that you bought a new Mac, that might be nothing...
Second, take your Mac OS X install disk and put it in the computer and boot it up holding down the 'C' key. That will boot your computer from the CD.
As soon as you have control over the computer after bootup, go to the Installer menu (I'm not sure that's the one... it's whatever's between the Apple and Edit, if there's even an edit menu there...) and select "Disk Utility"
When that loads, select the "Partition" tag and then select your drive. It should be pretty apparant how to setup the partitions you want from there. My suggestion is to reserve about 4-5 gigs of the drive for your Mac OS X and some apps, and then you could partition the rest anyway you want. Either by subject, client, product, whatever... I'm not into graphic design so I don't really know...
Anwyays, after you've set that up, click Partition and it'll erase everything and repartition it for you.
Oh, and make sure that your OSX disk is an install disk not an upgrade disk! You won't be able to reinstall OSX after you partition if that's so...
Anyways, hope that helps. I'm sure others have some things to add or change, so listen to them