When I asked a pro writer what I had done wrong with a story that was supposed to be a novel, but came to a natural end after 30kw, she said "You didn't give your hero enough problems."

A plot, basically, is a problem that you set the characters and watch how they solve it, throwing them more curves any time they look like solving it too soon to be fun. The problem can be outside - a murderer chasing them, an unattainable love, a magic ring they have to destroy - or inside - they're cripplingly self-doubting, they're born weak and deformed in a world that prizes strength, they've lost their memory - or best of all, some from column A and some from column B. Bujold, who I just referred to, said that her plot generator was "What's the worst thing I can do to this character?" Tolkien's characters are thin cardboard, but he could plot. Watch how he starts with a challenge that sounds as bad as it can get (break into the grimmest fortress in the world, carrying the one thing its ruler mustn't get hold of, are you serious?) then makes it steadily worse with one new setback after another till you really can't see how they can possibly win... so the moment when they do is like sunshine after a thunderstorm.

There are various plot templates that people swear by, but as far as I can see they come down to a few simple steps:

State the problem. Who is your protagonist, what's his/her problem(s), and (most importantly) why should the reader care? And state it fast, because most people want to be hooked in the first few pages, or they're out of here.

Protagonist confronts the problem cautiously, and finds it's bigger than it looked. Tries again, harder, and gets knocked down harder. Rinse and repeat until:

Protagonist appears completely defeated, everything has gone wrong, the farm is going to be sold, the lover has left, the dog has died, the murderer is chopping down the door with an axe.

By an amazing achievement which nevertheless was, with hindsight, implicit in the situation and the protagonist's nature right from the start (gods from the machine not required), at the very last minute they overcome all odds and win the day, slay the princess and marry the dragon and ride off into the sunset.

Try it!