While this will be seen as blasphemy on this board. If you're a beginner and just making a some beats, plugging those KRKs into your headphone jack via a stereo 1/8" to Dual XLR cable like this (http://www.amazon.com/gp/product/B000068OEP/
) will tide you over just fine till you're ready to step it up.
Yes an Audio Interface will sound better (depending on the interface) and yes it is important but so is having your room acoustically treated among other things. Its really about what your budget is at.
When you're ready to purchase an interface I highly recommend the Apogee Duet 2.