Well, first, make sure your runway is long enough (at least 7000') and that you wait for the "rotate" call before pulling back. I say this because you'd be surprised how many people forget to pull back on the stick, or try to take off from a 3000' strip. Won't happen.
If that isn't the problem, make sure that you have some flaps set (10 degrees is good unless you have a really long runway), and that your trim is set correctly. Usually, 5.5 degrees of up trim will be good, unless the FMC tells you differently (even if it does, or if you don't program it, 5.5 will get you off the ground). You can try the autothrottle too (make sure it's engaged, and then press the top left screw on the autopilot to put it into takeoff mode).
One more thing you might want to check is whether you are overweight. Some planes start up with full load and fuel, and this will be over the max takeoff weight. Take some passengers, bags, or fuel out and try again.
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