dimension X part one

OK, so, I'd correct the grammar, but that's already been done. You really need to take more care with it. So the main problem I have with this is that there is no descrpition at all. You need to describe the characters, as well, because if we don't know who they are, there's no chance of us being afraid for them.
At the end, when there are monsters attacking Samantha, would she really believe that they were monsters? I know that if I saw things like that, I'd think it was some freaky costume. This is why you need description, because right now, with none, they're not scary. So the reader is left wondering why Samantha is actually afraid.

Then out of no where these mosters started to come out of the ground.

*monsters.
This is where you really need a description. When I read this, no image comes to mind. There's nothing to let me imagine what's happening.
Also, you switch from the past to present tense in the middle, when you need to stick with one. To me, it seems more like stage directions than a story. The whole thing could do with more length and depth.
I do agree with Starrywolf, though. With more work, this could be very good. Sorry if I was a bit harsh!
-Anna

Firstly, I will fix grammar.

It was a normal day and Samantha was walking with Sebastian to school. "For some reason, I have a feeling that something weird is going to happen today," Samantha said.
Sebastian laughed and said, "You always say that. You gotta stop reading manga."
"Whatever! You're too slow. Come on and try to catch me," Samantha says as she starts to run.Sebastian tries to catch her but she disappears.
"Samantha, where did you go? We have to get to school!" Sebastian says.
Samantha looked confused and said, "what do you mean, where am I? Um... Hello, I'm right in front of you!" Waving her hands she says,"Are you blind?"
Sebastian then turns around and says, "Whatever,I guess she is skipping school". He runs off.
"Sebastian, you dumb ass, where are you going?" Samantha says, trying to run but her feet cant move. She yells," What the hell is going on... my feet aren't moving!" Then out of no where these monsters started to come out of the ground. Samantha, being scared, says," What the hell what are those creatures? Ah!They're coming closer! Help me! Someone help! I don't want to die!"

Some major notes:

You don't have to have a comma after every quote. Actually, If I'm not mistaken, you never have a comma immediately after a quote.

Every time the speaker changes, it's a new paragraph.

When you're using an ellipsis, only use three dots.

Remember capitalization.

Don't use more than one exclamation point.

Remember that you'll get more reviews if you try with the grammar. People find it easier to read if it's... readable.

Now for the story:

Something I found odd is that Sebastian just accepted that his friend, standing right next to him, had disappeared. If somebody standing next ot me vanished, I wouldn't brush it off as her skipping school. Especially so calmly.

And when the monsters came out of the ground, I was like, wait, what?! Describe the monsters a little, make them more realistic. I wasn't really scared for Samantha, because you didn't instill fear. You have to make it scary, like I said, describing the monsters or something.

This could become a really cool story. Just try on the grammar, and try to make the characters and emotions a little more realistic.