Stephen King doesn't plan his plots; at least according to his book, "On Writing." Perhaps that is more your style (and is what I like, too). King's approach is to write about a problem and do exactly what you said: imagine characters, and scenes, and get in the head of the characters.
The "Plot" is just the story line; you may need to learn some things about that, but basically (in this approach to writing) what you have in mind is the "Big Problem" to solve. You may have some vague ideas about how characters will solve it; for example "eventually Bill kills his best friend to stop him", or "I know Marcia will be raped, shot, stabbed and set on fire at various points in this story, but in the end she wins."
Those can change as your story develops; maybe Bill finds a clever way to neutralize his best friend without killing him. Maybe the gauntlet Marcia must soldier through involves a near drowning instead of being raped.
This approach is to first see the Big Problem, then see Characters and how they discover it, and what they then do about it (or because of it). Then write the scene, here is what they were doing (playing with metal detectors in a field); here is what they found (a buried Army duffel bag full of thirty million recent-issue dollars and a bloody knife).
Think about your characters; get into their head. Make them different (of course if they are friends or lovers they probably have some things in common). What does each of them do next? Of those, What is THE most interesting next scene that is going to alarm or provide some force to make other characters move? Are there hidden characters (who buried that Duffel?)
Don't fall in love with your scenes to the point you can't bid them a fond farewell; this approach can take some backtracking and discarding. But do remember that in real life, we cannot do that: Sometimes, what seems like "stuck" for a character is an opportunity for desperate action: Self-sacrifice, murder, deception, coming clean, surrender.
Don't be afraid of correcting course (or putting some foreshadowing) in earlier scenes: For example: Joe didn't tell Charlene the knife was bloody, he lied.
Also, the characters will take on lives and personalities of their own in your mind; so as you read earlier scenes you may feel like they are acting out of character, and rewrite a little to put them in their more developed character.
The plot will emerge as the characters take their actions to deal with it; just make sure the Big Problem is enough to motivate them until some sort of conclusion is reached.