tm,
Quick answer is 500'.
There are a number of different rules (1500', 500', 1000m, ability to land clear, and that's just for VFR - there's a different set for IFR!)
Even 500' isn't necessarilly the lowest you can go. You have to be 500' from any person, vehicle, vessel or structure (or something like that) so if you can find a completely isolated piece of land, you can fly at 10' if you want. (Not saying it's safe, but it might be legal.)
Don't worry - you'll have to learn all this for your air law exam! And you can ask your instructor for more clarification, too, if you want - that's what he's there for.
Take care,
FFF
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