I've written 2 full length novels, several short stories, and a hell of a lot of unconventional poetry.
That said, I can offer some advice. I assume you're writing fiction?
The first sentence of every story is always the hardest. After that let the story write itself. Never force it. You have to have that itch to get back to writing it. But before you get too far you need to think about a few things.
1) Is it a narrative?
2) Are the characters relatable?
3) What kind of conflict will they be dealing with?
4) Is it told through eyes of a certain character?
5) Have you written a description of all the characters yet?
6) Do you know where you want to go with the story? Have you thought about the beginning, middle, and end separately so you know what you want to tell the reader?
7) Is it engaging enough to keep a person reading?
8.) Can you describe what your story is about to someone who asks?
9) Are you writing just as a hobby, or are you thinking about publishing?
10) Can YOU relate to your characters? For example in my first book, the main character was based off of me. We share a lot of the same attitude and view of the world. The only difference is that my character in that story is male, and I'm female.
If you need more just ask and I'll help as much as I can. As a note to what @Withywarlock said, one of my books is currently on its 8th draft ans the other is on its 7th. More drafts will follow. Until it's perfect in my eyes, it will keep getting edits and rewrites. So don't get discouraged if you go to 7, 8, or 100 drafts. If you don't like it where it is, no one else will either.