My honest answer is...
A high level of ability in multiple vareid diciplines (Black Belt, National level amateur comeptitor etc). Best bet is either MT, Boxing, Kickboxing, Thai boxing or TKD twinned with either Wrestling, BJJ, Judo, Sambo, Savate or the equivelent.
Then you will need at least 5 or 6 amatuer fights with a decent record to earn a professional licence. (which will probably take a year minimum)
Then you will have to get a string of professional wins against guys who have probably trained longer than you.
All this before i have even mentioned that you'll need a very high level of natural physical ability, as well as a ridiculously good work ethic and mental attitude.
Don't be fooled by the likes of Tim Sylvia or Brandon Vera - At this point in the evolution of MMA no-one who isnt an elite, driven and naturally gifted athlete will get near the UFC.