I might be pretty harsh in my critiques of your sentences. Fair warning.  

"There is a world beyond ours, [Comma Splice alert!] it has been car[e]fully hidden out of our sight for hundreds of thousand[s] of years, until lila [should this be capitalised?] Migrinch bacmae [spelling: became] a whisper in our world."

My reaction: Mmm, interesting. I'd probably keep reading a bit, BUT! Comma splices and spelling errors make me put a book down. Get them right. One other thing: Your tenses are wonky. "It has been" implies that it is still hidden at the time of the telling, but "until Lila Migrinch became" says that it's not. I'd put it all in past: "it WAS carefully hidden." A better version of your sentence might be: "There is a world beyond ours. It was carefully hidden out of our sight for hundreds of thousands of years, until Lila Migrinch became a whisper in our world." 

"Past the rings of [S]aturn, through the x planet [P]luto, you would find a place on which humans had once evolved."

My reaction: What? Through Pluto? Do you mean past Pluto? I assume the x is meant as the Roman numeral. If Pluto was the tenth planet, I'd advise you to change it to "tenth." But Pluto isn't the tenth planet, it's the ninth. In fact, to be even more nitpicky, Pluto is not a planet at all. It's a dwarf planet. Do your research. I'd replace "you would find" with "there is". If you want to use "you would find," it would probably be a good idea to add something at the beginning, so you would have "If you went past the rings of Saturn." Also, "place on which" doesn't work, it doesn't make sense. Change it to "place where" or "planet on which." Once I've gotten past all that, however, the idea is rather intriguing. 

"I watch my mother with tears stre[a]ming down her cheeks, saying goodbye to the world she once knew.       

This is my first and last memory of my long[-]forgotten mother."

My reaction: Why do you have only one memory of your mother? What world is your mother saying good-bye to? I'll continue, sure. But: Who has forgotten your mother? You? I mean, I get that if you have only one memory of her she is pretty long-forgotten, but it sounds wrong, maybe because if you have a memory of her, she still isn't entirely forgotten. If it were me, I'd probably just delete long-forgotten, and get "This is my first and last memory of my mother." Simpler, less confusing, gets right to the point. That word doesn't really tell us anything we can't infer from context.

Now that we've got past all that, we get to continuing off from these sentences. I can't really help you here, for one simple reason: I don't know what's going to happen in these stories. You, presumably, do. What do you want to happen? Write that. Is there more to explain about the background/setting/premise/what have you of the story? Explain it. Always keep in mind that your first line is not set in stone. If your story changes, you can change it. Don't let yourself be tied too much to it. 

@ZNZ:  I might need to get you to edit some of my stuff.  Pwetty pwetty please???

@Hana:  I don't think you need to make them longer. A good author can grab the reader's attention with any size sentence ;)  and btw, you're new, right?

Oh, and as for the last question, 3,882,000.

1)  There is a world beyond ours; it has been carefully hidden from our eyes for thousands of years, at least until Lila Migrinch.

2)  Past the rings of Saturn, and beyond the ex-planet Pluto, is the place on which humans evolved.

3)  I watch my mother, tears streaming down her cheeks; she is saying good-bye to the world she knows.

     This is the only memory I have of my mother.

Here they are, edited and ready for use! I hope that's what you had in mind, especially #2. If it isn't, please let me know!