Writing the First Draft
When planning the opening scene for a story, there are two approaches you can take: a direct approach and a subtle approach. The direct approach is the option most writers take. The so-called action of a story begins immediately, and the reader is submersed in the world created by the author, usually alongside the center character(s). The subtle approach, though less often used, can actually be just as effective if executed well. It involves starting with a seemingly normal scene that soon reveals unusual circumstances, objects, etc. A story of this nature is often told in limited omniscient point of view (though not always), starting big and zooming in on a scene.
One you've chosen your method, it's best to get right to the action. And no, that doesn't mean that you have to start with the stereotypical action scene. As I stated previously, you can choose a rather normal scene with some oddities. The key is to start with a place where the plot is progressing and not with backstory; you can weave backstory in later. Movement should be your focus; as long as the plot is developing, your readers will be interested.
The other thing you want to avoid is commonly occurring scenes for the particular genre you're writing. For instance, if you're writing a mystery, don't feel that you have to riddle the opening scene with death. You could have an investigator studying an almost-closed case where he realizes something is wrong. You could have a convicted felon escape from prison with some inside help. You could have an investigator stumble on to what he thinks is a case only to have it be a bust. The point is, there are numerous ways to tackle a common genre without giving it a typical opening.
If you know the method you want to use and have an idea for a developing plot, there is only one thing left to do: write. The initial writing process is different for everyone. My personal take is to simply write and not overthink things. After all, what is written can always be erased or altered, but what isn't written won't always come back to mind later. The downside is that this makes for more editing later. Another technique is to write only what you feel is necessary in the scope of the whole book. This is certainly more difficult to do but reduces the amount of time you'll have to spend editing later on. Both strategies have their pros and cons.
The Editing Process
Once the first draft of your opening scene is staring you in the face and you're ready to tackle editing, be prepared to get down to the nitty-gritty. Here are some suggestions you might want to follow as you push through the editing process:
Once you've gone through the stages of writing the first draft and editing it, go through the editing process again. And again. And again. And again. Perfecting a book takes time and practice. It's very unlikely that you'll end up with the end product you want after only a few edits. A friend of mine (a fantastic writer by the way) is on at least round thirty of edits, and she's still ironing out kinks. But after reading the revisions she made, I can tell you it was worth it. Her story was transformed from great to phenomenal.