5th dnd – Does the fall count in my movements?

I am new to D & D. I have read some answers to similar questions to mine, but I wish further clarification.

Can I, as Winged Tiefling, fall on an opponent as part of an attack? And more precisely, is the distance I fall from the distance of my travels?

Example

I have a speed of 30 feet and I have 15 feet more than an opponent. I want to fall on him using my weight as a weapon. I then want to move 15 feet, cast a spell as an action, then move from 15 to 30 feet as a bonus action and throw a Cantrip.

Would the 15-foot drop, because of its involuntary movement, cost me some movement, thus removing the second part of my movement phase?