From my mentor of sorts, Doyle McKim.
Aside from providing the first steps of a long path, a beginning has one overriding purpose: to draw the reader into the story and to hold him. The following are guidelines that will increase the writer’s chance of doing that. There are five of them: The Hook, Time and Setting, Characters Present, The Vein, and the Conflict.
The hook should come in the first sentences. “The last camel collapsed at noon.” Or: “When I opened my eyes, I could see nothing but white.” Simple declarative sentences. Nothing is stronger. Then keep writing. Build the story. Do not stop for flashbacks, do not stop for memories though long blocks of introspection. Show where your character, or characters are, when it occurs, and what they are doing.
Establish the vein. Is it humor, drama, or romance? Your beginning is the promise of what is to come. The reader who thinks he is getting drama but finds it is light satire, will be unlikely to look for your name on the next book he buys.
Through all of this, provide the conflict, or enough hint of it to hold the reader. What does your character want? Why can’t he have it? What does he do, or intend to do about it? The conflict is the reason for the story.
If you plan on parallel plots or subplots, bend them toward the storyline in a way that lets the reader know there is, or will be a connection.