Cross country is a bad life choice...get a DH bike and let lifts do the work for you.
Do what you want, DH, DJ or XC. it doesnt matter as long as you have fun. all i can say is for xc do beginner first, if you start taking first then move up to sport. just start riding but never loose the fun because once your not having fun it will turn to shit. ride when you feel like it, if you start riding and you dont feel like going any more but you push yourself to then turn around. i also suggest getting a single speed and racing/riding ss instead. way more fun that geared bikes and you can smoke way more pot and ride because you dont have any gears to mess with.
Buy a Road Bike
Ride your road bike at least 3 times a week for 2-4 hours per ride focus on hilly roads
Ride you MTBike 1-2 times a week on technical tracks
Get a mountain bike with minimal travel suspension
Make sure your bike has some nice light wheels
Get clipless peddles and proper shoes if you don't have them already
Get some tight fitting clothes - look as Euro as possible
Shave your legs.
Thats how to win.