Originally Posted by Ebs
If I where to buy a mostly stock bike to learn on and putt around the trails with I'd look at:
Honda XR250 or XR400. They're heavy and slow compared to a modern 450 motocross 4-stroke, but don't require near as much maintainence, and they make tractor like power down low which is nice for cruising around the trails.
Kawasaki KDX200 or KDX220. Pretty popular 2-stroke trail bike that shouldn't be too much of a handful for a beginner.
And of course there are tons of less-than-fullsize bikes out there that are pretty nice for learning on, but once you figure everything out you'll probably want something a bit better anyway.
Cannot disagree on what EBS suggested, though, an XR400 can get really heavy at the end of the day.
First and foremost: protective gear.
Get a set of motorcycle boots before you set tire on trail - my boots have saved me from broken ankles and toes almost on a daily basis.
Buy a helmet that fits (means shopping in a store and paying a little more $$$) and a decent set of goggles.
Jeans and a light-colored long sleeve shirt will get you by for your first rides. When you do look at armor (since you're trail riding), I would suggest staying away from MX-style armor. You don't need "roost" protection, you need tree protection. Keep your eye open for sales on "661 Pressure suit" armor - that has saved my upper body almost as much as my boots have. I bang off of trees with my handguards (another good woods purchase) and the arm protection quite often - the bruises I save are worth the extra money.
And last, but not least, go with some people that know what they're doing and where they're going.
Shoot me a PM if you have any questions - I'd be happy to answer more questions and introduce you to Michigan's awesome ORV system.