For me, the most memorable fiction revolves around the characters who people it. Yeah, plot is fine, action and all, but what I remember more are the heroes and villains, the good, bad, and the ugly. Most of my own fiction begins with a character or characters that I want to see dance.
I have mentioned before that the old pulp writer advice was to chase your hero up a tree and throw rocks at him, and that's still the best way to see what your guy has going for him- or herself -- here's a protag, here is the problem, and ... now what?
Throw rocks, see what s/he does. Make the rocks bigger, lob more of 'em, and let him (or her) figure out how to avoid them, bam! there's your story. Give a reader somebody they like and then threaten him, they'll stay with you to see if he is gonna survive.
I've discussed the various sources of conflict before, and as long as there is incoming, your protag will get a chance to show who he was and who, in the end, he has become. And that, friends, is the essence of good genre fiction -- character change.
If he's exactly the same as he started, it's a yawner. He needs to learn something about himself, ramp his game up a level or two, and be different at the end of the book than he was in the beginning. What he saw and felt and heard and did needs to matter enough to make him a different -- and most of the time -- better person as a result. The dark moment of the soul, and then the light.
You'll know more about your hero and her sidekick, and the villain, and his sidekick by the end of the first draft, but there are a couple of tricks that will help you know more about them to hit the ground running at the start.
Do a short biograph of each major character. A page is plenty. In it, list his or her age, height, weight, hair color, eyes, any scars, and what their ethnic roots are. A bit of history -- where they were born, went to school, work, relationships. Family -- grandparents, parents, sibs, aunts, uncles, like that. Did they have dogs or a cat? Were they good students or troublemakers?
One of my favorite things is this: What were they doing on their twelfth birthday? If you know that, even if you don't say it, it gives them a depth they wouldn't otherwise have. Me, I often throw it in, because a pivotal event at that age can mark your character forever, for good or bad.
Set them up so that they had a life before your story starts -- and you know what it was, and that will come across between the lines. A word here, a line there, you reader will see that you know, and that matters.
Then, give your protag a problem. A big one. Life or death, for them, for others, and have it be something they can't just solve without a lot of effort and risk. The big problem will spawn smaller ones, and in my world, solving the small personal problems will solve the big one around which the story revolves.The small stories, the personal ones, are as important as the big one. More so.
The Empire wants to stomp the Rebellion and they are building a kick-ass Death Star to do it. Leia needs to stop it, and she sets it all in motion; Luke is coming of age; Obi-wan needs to teach the kid about the Force and he doesn't have much time; Han needs to find something more than money to live for; the Emperor needs to twirl his moustache and Vader needs to breathe heavy and crush throats.
(And the only person in the series who always seems to have a real sense of the right thing at the right time is Artoo, hands down the smartest character in Star Wars.)
To give Luke the impetus he needs, Obi-wan has to allow Vader to cut him down. For Luke to get a real shot at the Death Star, Han has to knock Vader for a loop. To destroy the Death Star, Luke has to trust the Force. Everything in that episode leads up to that moment, and that is the instant when he becomes a Jedi. Solve the personal problems, solve the big one at the same time.
Next movie, they all have a new set of problems with which to deal. Each episode has a different set of small problems that, dealt with, solve the big one.
People want to read about interesting people. They can be dogs or aliens or sentient trains, but they have to be somebody to whom an audience can relate; can root for, or against, can cheer or boo. Give them that, you'll do fine as a story teller. Fail to do that, probably you won't do so well.