Most importantly, in something like a 152 on takeoff, it would be near impossible for it to flip over back on itself. They have very docile handling, and if it was near a stall, your instructor a) shouldn't really be conducting introductory flights and b) would correct it. The stall doesn't *just* happen, there's plenty of warning from the aircraft and its systems.
Above all, don't be nervous. Relax and enjoy it. Are you thinking of doing your license?