THE PLOT: The plot of a story may be described in three simple terms:
1. Object
The protagonist (main character) wants something so badly that he is willing to risk everything to get it, even his life. The antagonist (villain) wants to stop the protagonist from reaching his goal. The antagonist may be a human or, in some cases, even the climate--man against man, man against himself, or man against nature. The interaction between the main character and the villain creates problems which must be solved by the protagonist in one way or another in order for him to achieve his goal.
2. Obstacles - Problems, Problems & More Problems!
Without obstacles there is no plot. Give your character a GIGANTIC problem and obstacles to face as he tries to solve that problem. The decisions he makes to solve each obstacle is the plot. The complexity of his problems depends on the age level you're targeting. Give your character choices--right choices or wrong choices. And when he finally thinks he can solve his problem (or reach his goal), give him a bigger one. Use the universal law of 'Cause and Effect': if he does this, it will cause that to happen. The idea is to figuratively knock your character down and kick him when he tries to get back up.
3. Outcome - Climax and Denouement
There must come a point in your story--after your character has done everything in his power to make things work and is down as far as he can go with little hope for recovery--when he reaches a 'point of no return' and MUST do something. His choice may be right or wrong, but it must be his own choice (not the author's). The choice he makes should be a surprise to the reader, although it may have been foreshadowed from the beginning. A hint here, another hint there. A reader should be surprised, but then shake his head and think: Yes, I see it now. That was the only thing he could do.