How does a nerve impulse work and reset itself?

In the resting state, the inside of the neuron's axon is negative about -70mV. It is negative because it has more Potassium inside the axon. So the concentration gradient of pottasium is more inside. And the concentration gradient of Sodium is more outside.
When a action potential arrives in the axon. The inside of the cell (-70mV) goes up by +100mV and become +30mV.

FAMIILARIES YOUR SELF WITH THE ANATOMY FIRST.
Basically Dendrites receive impulse, carries it to the axon (body) and then out the synapse. The synapse then links again with another neuron dendrite or just a muscle or something.

So action potential begins in the dendrites of the neuron.
It receives and impulse or message from the brain or from other neurons.
The impulse depolarizes the membrane of the cell so that the inside becomes positive (30mV) and the outside becomes negative.
It only can become positive when sodium rushes inside the cell making the cell positive on the inside. It does this because it has 'voltage-gated sodium channels' which work because of an impulse.
The cell then releases the sodium from 'leakage channels' back to the outside of the membrane, make the inside negative (-70mV) again and the outside positive. Remember when it is negative inside it is at resting state.
But when the inner part of the membrane of the cell is positive with sodium it creates a continuous propagation along the cell. Meaning like it contiously changes the voltage along the cell until it reaches the end of the cell. So you can imagine it like 10people holding hands side by side. The person on one side starts with the wave (action potential) and this is will make the next person do the wave again because they are connected. And this wave (action potential) keeps going until the very end of the line (cell). In the case of a cell its the synaptic terminal where it will release the impulse with neurotransmitters.
And basically when ever the person finishes doing his bit of the wave he will go back to resting his arms. Same with the cell, when parts of the cell have carried out the impulse they will go back to resting state, which means negative on the inside and positive on the outside.

I hope that helped buddy.