While every school is a bit different, most are structured along the same basic lines. The first part of the course centers on book-work; learning how the hours of service work, filling out logbooks, memorizing the federal laws that truckers operate by and other information that new drivers need to know. The second part involves backing and driving around a closed course. Students will learn at the very least straight line backing, alley docking (backing into a space from an angle) and parallel parking. The third and final portion generally involves an instructor taking a group of students out onto public roads for short trips of 100 miles or less.
Each state sets its own licensing requirements, but most follow along the same lines. Most people can acquire a CDL if they're at least 18 years old, possess a valid drivers' license, pass a physical examination, don't have any suspensions/revocations in any state, don't possess a valid license from more than one state and haven't been convicted of operating a commercial vehicle while intoxicated within the last 24 months. You must obtain and maintain your CDL in your state of legal residence.
Traditional schools fall under one of two basic categories. Independent driver-training schools like MTA and Roadmaster are typically the quickest but most expensive route, and generally offer some kind of driver-placement program. College-based schools run by community colleges, universities and adult education centers might be a bit less expensive, but classes may fill up quickly and may only run a few times a year. A private school is probably the safest bet if you're looking to get on the road quickly and are willing to spend the extra money.
Company-run schools have been gaining in popularity since they first emerged in the late 1990s, and for good reason. Company schools carry most of the advantages of private schools, but most offer guaranteed job placement within the company pending graduation -- something few independent schools can promise. Company-run schools also offer continuing education in the form of months-long driver training following graduation. The new driver rides with a driver-trainer for a number of months following graduation, and the trainer releases the student once they're confident that the student can safely operate a truck alone. You don't necessarily have to go with the company after graduation, but most companies offer to pay off part or all of the course if you stay with them for a year or two after obtaining your CDL.
While you may be able to obtain a CDL at 18 or with a dozen points on your license, simply obtaining a CDL doesn't mean you're hireable at a given company. Ultimately, a company's insurance provider determines who it will or will not cover. Don't be too surprised if you find that the company of your choice only hires drivers over 21 or 25 years of age. Some companies won't hire drivers who live in certain areas (like Florida or North Dakota) simply because they do very little business in that area and won't be able to get you home.