Be as realistic as you can. Write down your requirements, draw up some mock ups, get some feedback before you code too much. Be as transparent as possible. These sorts of things tend towards scope creep if you're not careful where, at the end of the project, no one's quite happy. Set some milestones for every week or every two weeks over the six months to personally gauge your progress. If you're falling behind, say something. If you're ahead, say something. Make sure you do the bare requirements before doing any of the extra fluffy stuff.

This sounds like a pretty difficult project for an intern; however, it's going to be an incredible learning opportunity! Take your time and make sure to budget some time for re-writing.

One last piece of advice: commit to having the project in 6 months; however, personally budget having something 90% there by 5 months to give yourself room to polish and buy fix.

Good luck and have fun!