I agree with Drunken Chicken, as a beginner to off-road, I'd ride w/o the clipless and then once you're comfortable go back to them.
Other beginner mistakes:
1. Locking your elbows and knees. Stay loose, use your body as suspension.
2. Death grip on the handlebars. If you are getting forearm pump, you're gripping too tightly. Keep your hands relaxed. Try resting your thumb on top of the grip instead of wrapping it around.
3. Not looking far enough ahead. Try to focus on the trail 10-12 feet in front of the tire, not 1-2 feet. Your brain will remember what it saw.
4. Look where you WANT to go, not where you DON'T want to go. If you stare at a rock in your path, you WILL hit it. Concentrate on your preferred line.
5. Drink plenty of fluids. Beginners usually don't stop to drink enough fluids and they get dehydrated and tire too quickly.
6. Do a quick once over of all your nuts, bolts and quick releases before you hit the trail.
7. Don't overinflate your tires. Max for off-road use should be 40 psi. I start at 45 psi and every time I stop I give my tires a quick little blurp of air until I find that "Sweet Spot" of tire pressure. It's usually in the low 30's for me.
8. Bring a buddy! Nothing better than sharing your experience.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger