The schooling you are required to take isn't that cheap, last I heard (a couple years ago) it was 6 or 8 grand, also you won't find anyone that will insure you until you are 23, and even then some places may turn you down until 25. I've known people to get LUCKY and get in earlier, but it's rare.
It's a stressful job, if you ever have road rage tendencies I would suggest against it, but it depends on what kind of driving you are going to be doing. Local farm stuff wouldn't be bad at all since you won't be in high traffic areas, and you would be home most if not all nights. If you go over the road, keep in mind that you will be gone for several weeks in a row out of the month. It sucks not being able to see your friends and family for that long.
Other drivers tend to have some sort of disrespectful attitude towards truck drivers, or they are too dense to respond sensibly. I knew a guy that used to drive that said he got in a wreck (that he was cited for) because someone had been in his blind spot for several miles before he decided to switch lanes. Never even knew the car was there. There are also people that go road rage against trucks because they are going slower, even though in some states, like Ohio, the posted speed limit for semi's is lower than for passenger vehicles (which is an unsafe law, IMO). There will be times when you find yourself overloading or driving longer than you are legally allowed in order to make it somewhere on time, because your pay generally is based off of either how many miles you go or how much weight you haul. You don't want to get caught overweight or with a logbook thats not true, both carry heavy fines. Also, if you have CDL it doesn't take much in the way of tickets to lose your CDL, even if you get ticketed when only driving a car.
The positive sides are that you get to see the country, and you have a lot of freedom. If thats what you are into, then great go for it, but this is a job that's definately not for everyone. I wouldn't mind having CDL just because you never know what situation you may find yourself in, but I personally would never drive a truck for a living, I would be miserable.
The pay is pretty good, my friends that did over the road were pulling in around $500 a week, and they were fairly young. There are also good ways to move up in the industry, if you don't mind maintaning your own truck, owning the truck you drive can bring in good money, also if you find a driving partner so that you never have to stop except for drops fuel and food, that would mean even more money.
Good luck with your decision, this isn't an easy one. I recommend you really think this through and talk about it to the people you care about, as it won't affect only you.