Tightening cast: John, 11 is the main character. In scene 3, a policeman comes to tell him that his mother was hit by a car. In scene 5, a social worker comes to take him into foster care until his mother recovers. If we make the social worker the policeman, that's one less character we have to write and one less character the reader has to remember.
Tightening plot: Professor Cornbluth is strangled in his kitchen, using mountain-climbing rope. But the rope isn't a special clue about the murderer -- it's just something Cornbluth had in the house. So replace the rope with a dog-lead snatched from behind the kitchen door and that's one less piece of confusion in the plot.
Tightening story: Say there's a subplot in which Laura's mother divorces her father. In scene 23, Laura's mother calls her to say she's thinking of leaving Laura's father. Laura tries unsuccessfully to argue her out of it. In scene 27, Laura's mother arrives on her doorstep, asking for a place to stay. If nothing else has happened in the subplot in the meantime, it may work better to delete scene 23, and just keep scene 27.
Tightening scene: Scene 12 opens with 500 words describing the weather, but it's just a mood-setter, and has no bearing on the characters, situation or subsequent plot. Cutting it back to 100 words may yield the same mood and spare the reader 400 words of tedium.
Tightening text:Jane was distraught. She paced back and forth, and didn't know what to do. Her heart hammered, sweat beaded her brow and her palms were clammy. "What shall I do? What shall I do?" she wailed. But she had no idea.
There's a lot of redundancy in the paragraph above, and a mixture of showing and telling. We could replace with:
Palms clammy, Jane paced, her heart hammering four beats with every step. "What shall I do? What shall I do?" she wailed. But no ideas came.
Hope that helps.