Do you have any major health concerns or are you missing some fingers or toes? Getting a medical waiver for some health issues is possible but not for everything. You want to think about this first so if you know you have major issues then you may want to learn more about the DOT requirements before spending money on training.
If the DOT exam should not be an issue the next step is learning about the different methods of becoming a truck driver. Do you want to attend a 3 week truck driver training program or a longer one. There are pros and cons to each, so understand what you will be getting yourself into.
During your truck driver training you should get some behind the wheel time. A better school may give you more time whereas a lesser school may not. After you have completed the school training portion you will likely work for a trucking company who assigns an experienced driver to work with you for awhile.
Learning a good base of driving skills is important to get at the truck driving school you attend. You do not want to solely rely on getting that type of training from a trucking company so make sure you check out the schools of interest to see how much time they will give you driving.
Getting a lot of real world practice driving, doing vehicle inspections, hook and trailer drops, etc is important. The books help, but getting your hands dirty learning the needed skills is important.
If the instructors at a school never worked very long for a real trucking company you may want to watch out. There is only so much you can learn from books and some of the most important skills will come from experienced trainers.
Finding a school where safety is considered very important is key. You want to learn good driving habits and safety to be a professional truck driver.