Answer:
To be honest it really depends what kind of genre you want; I know it's not a book but a genre really helps so let me give you an example.
Fantasy: Your main character is in the woods with her best friend and then a troll comes out and attacks but then a knight comes and saves them. (I know, a bit lame but it works when you put all the cool stuff ;)
Mystery: Your main character is in an ally way looking for one bad person who's been killing a lot of people and the character finally catches them. (This is kind of vague but I hope you can see where I'm going with this)
See how both are really different but could turn out into a really cool story? I hope this helps a bit.
PS. I love to write stories so try and have fun with it :)