One thing I've learned as I've slogged my way through this whole "word thang" (as my husband calls it), is that story and plot are two different things. Story is what the novel is about, while plot is the series of events that occur to illustrate the story. Story is, but plot does.
Coming up with story ideas is pretty easy. Coming up with a plot -- not so easy. But there are things you can do to make it a little better.
Every story has a beginning, a middle and an ending. Think about how your story starts, how it ends, and what things might happen to connect those two events. Back in the Start Me Up post, I created a story about a boy and some Martians, so let's play more with that and see what we can come up with for a plot.
First, get the basic logline to set the general premise.
On a school field trip to the moon, a boy is captured by a lost tribe of Martians who will eat him if he doesn't escape.
Look at the beginning first. The hero is going about their business when suddenly, something happens and they have a problem. This is the inciting event that causes the rest of the story to happen. No matter what the problem is, something occurred that made the hero think, "gee, I have a problem I need to fix." It may not have anything to do with the actual story problem, though it'll probably be connected in some small way. Even if it's just a situation that caused the hero to be in the right place at the right time to have the rest of the book happen.
In this case, the boy, Jack, is on a school field trip to the moon. He needs to find and be captured by Martians, so what might he do that puts him on this path? Notice I said "he do" and not "happens" here. It's easy to have something happen, but then you force your hero to react instead of act. Acting is where the story drive comes from, and that's what keeps readers reading. So, maybe Jack is bored by the teacher's lecture, and he wanders off to look around. He finds some old ruins buried in the sand. When he touches the wrong thing, a door opens and he finds steps leading down.
Now that we know how the story starts, let's look at the ending. He has to escape or be eaten. Sometimes you know right away how your hero is going to solve their problem, but other times you only have a vague idea. Since we know Jack is going to be eaten if he doesn't get away, let's say he escapes by sabotaging the oven and blowing a hole in the side of the kitchen. It's okay for an ending to change as you develop a story. At this stage, it's just a guideline to give direction. You could just say "he gets away" if you wanted for now.
Now for the hard part: connecting these two points. This is where writing styles will start to differ. A free form writer might not want (or need) to know anything else beyond the start and end to write the novel. They prefer to let the characters develop and see where they take them. A structured writer might want to know every single thing that happens before they ever wrote word one. Then there's the in-between writer (like me) who likes to know the major events so they have a solid framework to guide them, but lets the characters and story develop organically. All are valid. Do what feels right to you.
Start asking yourself, "What can go wrong?" and "What does he do next?" Look at it as a big connecting puzzle. Jack is going down the steps into the unknown. What can go wrong? Start listing things. He can slip and break his leg. He can go deeper and get lost. The door can slide shut behind him and he's trapped. One of the things on the list is bound to make you say "ooooo" and sound like a fun thing to do next.
I like, the door slides shut and traps him. This forces Jack to move forward, thus moving the story forward. It also raises the stakes. He's trapped, and he'll be left behind if he doesn't get out. (And who doesn't have that "I'll be left behind" fear or memory to make them connect to such a situation). What does Jack do next? He probably will try to find a way out. As he looks for a way out, what can go wrong? (see the pattern?)
He gets lost. He falls through a hole in the floor. He sees light and hears odd voices. The goal is to find the Martians, so which best leads Jack in that direction and provide more options for trouble and intrigue? Let's give him the lights and voices, as it makes a reader curious who has the light and what they're doing there in that secret underground place. It would also be something Jack would also be curious in, so you have your reader and hero in sync. Jack heads toward the light and voices. So what happens next?
Was. Rinse. Repeat.
And keep asking, "what happens next?" and "what goes wrong?" They'll never lead you astray.
Even if they will lead your protag astray.