When I was in the spaceship, a hundred of aliens came.

I was i scared and ruan away.

I was really scared, because behinde mye was a very big alien.

She shouted "rktlkr...HELP...rkler" After that call I was really woried abut my mum.

tThere was a really big door.

But then the girl helped me and we could open it together.

Behind the door there stanood Mr. Rich and he said " Welcome back to reality.

It was only a hoax.

I'd say nightmare, hoax is more like a trick, a deception.

I think it (what?) was a hoax so I didn't go back, because I thought Targon is the best planet.

You can't start with "it" like this, you need to introduce what the speaker think might be a hoax.

When I was in the spaceship, a hundred of aliens came.

I was i scared and ruI ran away.

I was really scared, because behinde mye was a very big alien.

She screamed " I found the exit.

Oh no let uLet's run."

"Oh no is a bit weird here with the previous sentence, as it makes it seem like the thing the girl is concerned about is finding the door.

"Let's" is technically an abbreviation for "let us" but it sounds really old fashioned and/or formal to use the full version, which doesn't fit the situation

She shouted "rktlkr...HELP...rkler" . After that call Ihearing that I was really worried abut my mum.

tThere was a really big door.

But thenLuckily, the girl help me and we could open it together.

It's grammatically fine, especially in a semi-informal style like this, but it sounds awkward to start multiple sentences in a row with "But"

Behind the door therewas standing Mr. Rich and he said " Welcome back to reality."

You must stay on our crowded and polluted eEarth."

"I was happy but I also worried about the future of the eEarth.

I thinkought it was a hoax so I didn't go back, because I had thought that Targon iwas the best planet.

Whilen I was in the spaceship, a hundreds of aliens came.

A hundred aliens=Pretty much around 100 aliens (give or take a few, it could be 90 something or a bit over 100)
Hundreds of aliens=Many hundreds of aliens (200, 300, 400, etc.)

I was i scared and ruan away.

[Then,] I thoughtrealized the girl was right.

I was really scared, because behinde my was a very big alien was behind me.

Alternate (To help enhance vocabulary_: I was frightened, because a humongous alien was behind me.

So I ran awayI kept on running.

Not gramatically incorrect or anything. However, since you've already said "I was scared and ran away", it doesn't need to be reiterated that the character is running. For narration and prose, it's better to write something that continues the tension.

Then, I saw the girl and I ran to her.

Alternate: "Then, I saw the girl and ran toward her."

She screamed " I found the exit.!"

Oh no let us run.Hurry!"

In English, the word "hurry" conveys that urgent need to do something quickly. Even as one word, in context it means to quickly run (away from something or toward an objective)

And we ran and ran.

Alternate: We kept on running.

Suddenly, my mum called me.

She shouted "rktlkr...HELP...rkler." After that call I, I was really woried abut my mum.

tThere was a really big door.

But then tThe girl help me andso that we could open it together.

Behind the door there standstood (a man named) Mr. Rich and he said, " Welcome back to reality.

You must stay on our crowded and polluted eEarth."

Alternate: You must stay on this crowded and polluted Earth."

"I was happy, but I also worried about the future of the eEarth.

When I was in the spaceship, a hundreds of aliens came.

I was iso scared andthat I ruan away.

I was really scared, because behinde mye was a very big alien.

So I ran away.

Then, I saw the girl and I ran towards her.

She screamed " I've found the exit."

Oh no, let us run."

And we ran and ranway for a long time.

Suddenly, my mum called me.

She shouted "rktlkr...HELP...rkler" After that call I was really worried about my mum.

tThere was a really big door.