We can protect our nature with following things.
> We can protect our nature by following these things.

Note: The better preposition to use is "by" instead of "with".
The preposition "by" is used to show the person or thing that does something.

First we have to use a cup when we brush our teeth because we can cut down our importatn water.
> First, we have to use a cup when we brush our teeth because we can cut down our water usage.

Second we must not throw away garbage anywhere because cause our soil can't disposal plastic itself.
> Second, we must not throw away garbage anywhere because the soil can't decompose plastic by itself.

Note: decompose is the better term to use in this sentence which means, to decay, or to cause something to decay.

Lastly we always have attention to the environment.
> Lastly, we should always pay attention to the environment.