You become a personal trainer by studying under one of the training associations (which means taking home a large 300 - 600 page manual and studying it from cover to cover) and then taking a test at one of their facilities.
I'm studying right now to be an NSCA trainer (one of the most prestigious certifications, as is my understanding) and it's pretty tough. Of course, I imagine if you had enough free time you could get through the book and take your test in 1 - 2 months. It'd be tough. Here's a listing of some of the chapters:
I. Muscle Physiology
- Muscle Microstructure & Macrostructure
- Sliding-Filament Theory of Muscular Contraction
- Types of Muscle Action
- Force Production
II. Neuromuscular Adaptations to Conditioning
- Neuromuscular Anatomy and Physiology
- Motor Unit Recruitment Patterns during Exercise
- Role of Proprioceptors in Learning Physical Skills
- Neuriomuscular adaptations to Exercise Conditioning
III. The Biomechanics of Resistance Exercise
- The Musculoskeletal system
- Human strength and power
- Sources of Resistance to Muscle Contraction
- Power Ouput During Resistance Exercise
- Movement Analysis and Exercise Prescription
- Applying knowledge of the Biomechanics of Resistance
IV. Bone, Muscle, and Connective Tissue Adaptations to Physical Activity
- Adaptation fo bone to exercise
- Adaptation of muscle to exercise
- Adapatation of connective tissue to exercise
V. Bioenergetics
- Adenosine Triphosphate
- Biological Energy Systems
- Substrate Depletion and Repletion
- Bioenergetic limiting Factors in Exercise Performance
- Oxygen uptake and Anaerobic Contribution to Work
- Metabolic Specificity of Training
- Applying Knowledge of Bioenergetics
Then it goes on for another 30 chapters. Getting to the end fo Chapter 5 will get you to page 80 out of 540 pages. It's pretty intense.
Other certifications may be less tough, but I can't really tell you about those.
As far as pay, it's pretty good. I think you can expect to start at least at $20 / hr and make as much as $50 / hr when you're at the highest level (or more if you freelance). Pay increases depend on the gym - but generally have to do with your level of certification, how many supplementary certifications you've received, and how long you've been a trainer.
The toughest part about being a trainer is getting clients. If you work at a gym, it's not so tough because they come in all the time, but you might not always have steady work. (If you're trying to be a GOOD trainer, the toughest part is staying abreast of all the research in sports training, nutrition, and supplementation).
You should totally go for it, though.