Get a 250 4 stroke and learn to ride the wheels off of it.
Take it to MX vet tracks, enduros, harescrambles and just start seeking out the nastiest trails you can find.
that's the bike that will fight you the least and let you learn the most.
Just make sure you're learning the right things. Either get some of Gary Semics DVD's or some of the dirt wise ones and just really practice getting your body postion right. Elbows up, good central body position balls on the pegs everything else will follow off of that.
And if you really like it start training now for PT. Moving a big 9x0 around fast is going to take allot of strength and allot of skill.