The term "hot" can mean different things to different people. So may thing a car with a heavily modified engine is hot. Others the exterior customization is hot. It really just means a customized car that reflects what you like.
Start out with a good car, or a very customizable one. Popular upgrades include a new audio system, wheels, better lights, paint job, new exhaust, low-profile tires, spoilers, stickers, window tint, and racing bucket seats.