While Rocket League features hundreds of car bodies, they all fall into a small number of hitbox categories: Octane, Dominus, Breakout, Hybrid, Plank, and Melee. Octane has the tallest hitbox and is the most popular choice for its well-rounded handling. Dominus is flatter and longer, favored for powerful flicks and precise touches. The actual car body is cosmetic within each hitbox type — all cars with the Octane hitbox play identically. Pick whichever car looks best to you from your preferred hitbox.
The core mechanics of Rocket League build upon each other in layers. Ground-level skills include driving, power sliding (hold the powerslide button while turning for sharp turns and recovery), basic shooting, and positioning. Aerial skills start with simple jumps and progress to double jumps, fast aerials (boost and jump simultaneously), and eventually full aerial control for hitting the ball high in the air.
Advanced mechanics include half-flips (quickly reversing direction), wave dashes (landing with a dodge to maintain momentum), dribbling (balancing the ball on your car's roof), and flicks (popping the ball off your car with a dodge for powerful shots). These techniques take hundreds of hours to master but dramatically increase your effectiveness.
Rotation is the most important positional concept. In a standard 3v3, your team should rotate through three positions: one player challenging the ball, one supporting nearby, and one back defending. After making a play on the ball, rotate to the back position so a fresh teammate can take over. Cutting rotation (jumping in front of a teammate who's in better position) and double-committing (two players going for the same ball) are the most common mistakes at every rank. Good rotation beats mechanical skill through Diamond rank and beyond.