How can I stop my thread from bunching when using a sewing machine?
Are you absolutely sure the upper thread is threaded correctly, especially through the tension control knob? (I would go ahead and re-thread the machine, since it's cheap to do.)

Other things to make sure of (most of which you've already done, but just for completeness' sake):


The presser foot is fully down (on many machines, the tension isn't engaged unless the foot is down)
The tension control knob is set correctly.
The bobbin is inserted correctly
The bobbin thread is wound in the correct direction
The bobbin thread is raised correctly (with the presser foot up, pull on the bobbin thread: it should unspool freely with very little resistance)
The needle is sharp and straight
The feed dogs are engaged/the bobbin winder is not turned on


Also, something that helped an old finicky machine I used to have was to clean out the bobbin area - the accumulated fuzz seemed to make it more prone to tension issues, somehow.

If none of that helps, I'm afraid it's time to visit the repair shop.