Why my Mac OS X Startup Disk used space is around 30 GB
Ok you've got iLife for free, and installed which means you have:
Pages, Numbers, Keynote, iPhoto, GarageBand.
I suspect that this adds upto just under 5GB.
Delete them from the application folder if you don't need them.

This is the link to grandperspective, it's free and really great.

And as I said in the comments Xcode is big 3-5GB, the iOS Simulators are big.

If you are going to develop in the start-up partition, I would get your development environment set up now and then afterwards add the 2nd data partition in.  

Oh and I forgot the sleep image - if you have 4GB RAM then you'll have a sleep image that mirrors the memory.  If your Mac battery dies when you power it up it recovers the exact state it was in when it shut down. It's the same when it sleeps.  If you have 8GB then you'll have even less drive space. 

Something to bear in mind when creating bootable partition.

This can be turned off, but not a great idea