Underground Writers Network Forum Writing my first novel... HELP..

Nice to meet you all. I have had this wonderful idea stuck in my mind for a few weeks. Thinking it was just a passing thought, I let it be but I havent been able to get it out of my head so here I am ready to start writing! I have two character sketches so far but have kept changing them.  I could use any advice on starting a novel and how to get going. I really am looking forward to my idea coming to life! 

Thank you!