Lot's of cardio... Start running, swimming, and lifting in a fashion where you're doing it constantly and you feel like you're going to pass out afterwards. Guess what? After you wake up from passing out, get ready to do it again!
Now on what styles to train in... Can you let me know what's in your area? That might give me a better viewpoint to suggest something.
And as was posted above. If you're serious about fighting, if your trainers are worth their years in experience they will know when you are ready. I'm just going to forewarn you that if you're doing this as a part-time thing (and not a lot of people can start training in MMA full-time), it may take longer than you initially hope.
It doesn't matter if you agree or disagree
... As long as I don't bore you and I spark a moment of thought, my goal is achieved
Queng leon queng tigre ecu tacacut, queca pa? - Pampangan Mandarigma Motto