Wanted to ask a few questions since I am a beginner and need a lot of advise=)
I don't have a great guitar amp , actually it's pretty crappy. At the moment I use my guitar with my Mac Pro and two krk rokit 5 studio monitors and the audio interface I use is the Apogee Duet.
Is this a decent set up or am I missing out on the quality of the guitar without an amp?
Keep in mind I use garage band a lot for learning=)

Also are studio monitor safe to use with a guitar or could they be damaged?
Thats a great setup, the Duet is a great interface, and the Rokits are good monitors too.
Not sure about the last bit though, what do you think you're missing out on without a good amp? Are you using modelling software? Because there are some very good software modellers out there, and for versatility in a practice rig you really cant beat good modelling.
I'm guessing you're using Garage Band for amp sims? Its sims are OK, but there is better stuff out there. For practice its fine though.

The monitors will be fine for use with the guitar as long as you're running them from the computers/interfaces output. Running an amp into them would obviously be bad.
Monitors are supposed to have a near flat frequency response, so you need amp sims/cab sims for it not to sound like ass.

I've got guitar rig 4 and amplitube 3, pretty high end software, neither of them can hold their own to a real amp imho.