Seen a lot of good answers here, but still not sure how directly they're addressing the concern of you 'spoiling' your story. IMHO there are two aspects to this; Structure/planning and descriptive style. For structure, Try thinking this way:
- the definition of a 'bad' or 'spoilt' story is one that doesn't 'go' anywhere or loses interest (this doesn't apply to non-fiction, obviously).
- Therefore, you need to find the most interesting bit of your story (the bit that really makes you want to write it) and make sure it's somewhere near the end - even if chronologically it was near the beginning of the narrative.
This should give two possible outcomes; if the climax is chronologically late in the narrative, you'll have a basic 'quest' structure that starts with a main problem of some kind, probably adds some challenges/confusions along the way and climaxes once the quest is achieved - there are hundreds of examples I could cite here. If however the climax is earlier on chronologically, you get a much more zany structure that's a lot rarer - the example that springs to mind is The Snake Pit by Mary Jane Ward, in which the beginning is confused and jumbled but the first key climax is reached once it becomes clear that the narrator (it's in the first person) is in a Mental Hospital recovering from a breakdown (spoiler, sorry).
If you can get all that roughly planned out so you have your story in bullet points, in an order you'd want it to be read (NB this does NOT mean it's the order you'll write it in) then you're well on the way to having a really good story. The second aspect to the story, descriptive style, is purely a matter of personal preference and will be unique to you, but probably influenced heavily by what you read - in my opinion, I like to read stuff that infers the obvious details but leaves them unsaid or says them in a roundabout way (any other fans of P G Wodehouse out there?!) but have a look at what writers you like, and see if they use a lot of metaphor/simile, particularly long/deep descriptions or short succinct ones, that sort of thing? For maximum effect you'll want to vary the style a bit depending on which bit of the story you're writing - if you think of it like a film, put the long full descriptions for the 'slow motion' bits and the short snappy stuff in the action-packed fast sequences. Or, and much more effective if you can, just describe everything however it occurs to you, then skim back through it and see if it seems the same when you read about it as it did when you were just thinking it. Strangely, this often makes a lot of difference...
Finally, just ignore any advice about rewriting or not. If you as the author feel something should be re-written, then by all means do so. If not, don't bother. As @Ingolifs answer mentioned, Tolkein's master work was written in little sections, many of which got edited individually at various different times. And if I'm honest, this answer got partly re-written before I hit the 'post' button!!
Most of all though, you have to enjoy writing it. If it was fun to write, someone will almost certainly find it fun to read. Same applies for if it was inspiring to write, or sad, or practically any other emotion you want to invoke in the reader.