Begin at the beginning. Since I'm a draft writer revision seems a little easy to me since I always begin at the beginning but knowing what happens in between and at the end helps make the beginning better. Though I will admit I do struggle over the first few paragraphs. Part of this is because I want to set the tone of the story and part is that unless I have a good start, I can't get to the end.
When looking at the first few pages of the story there are questions to ask yourself. As a judge for many contests for unpublished writers and sometimes published ones I often find that the first paragraph is wonderful and there is a hook that draws me to wanting to like the story then several things happen and those are the questions you need to ask yourself about the first three pages of your story.
Is the opening bogged down with lyrical descriptions of people and places, or are the characters talking but nothing's happening or are they bogged down in everyday matters that lead no where?
Does the story begin here or do you have to go back and spend pages setting up what went before because you can't be sure where you want to start?
Have you devised an elaborate set-up that takes a lot of words but the rest of the story doesn't fit the wordage in the beginning?
Is the opening interesting and is the purpose clear?
Is there meaningful action rather than a tempest in a teapot?
Do you need a prologue? Now prologues must have a direct effect on the story. Some prologues work and some don't Readers remember the first thing they read and if there's no connection to what happens in the story, you've wasted words.
If you have planted some object in the first few pages has it been used. Say you describe the grandfather's clock in long detail. Does this clock have something to do during the story or if it no more than a prob that could be described in a word or two. If you talk about a gun, someone should use it.
Does your beginning have anything to do with the end? This may be the most important thing. Suppose your beginning is full of action but this action ends by chapter three and the story now turns to introspection. Then the end becomes slow and dragged out.
So my advice is to finish the book and then you can go back and work out the kinks beginning with the beginning.