You need to check with your state's requirements for obtaining a license, and what class of licenses are available.
Generally, with the states I am familiar with here in New England, there is an E-2 Journeyman and E-1 Master. To be able to be eligible to sit for an E2 test usually requires a 4 year apprenticeship, which includes On the Job Training working with under an E2's or E1 supervision, and classroom training, that includes algebra, trig, blueprint reading, and of course Code classes.
Sometimes if the math classes were taken during high school they can be waived.
Each state has their own list of hours that must be documented OJT and classes.
An E1 can do the work alone, an E2 must work for or under an E1, and apprentice must work under either.
Some states will let someone who has not served an official apprenticeship program to sit for an E2 test, depending on previous, documented, experience, say coming out of the military, or has worked on an industrial plant.
The primary aspect that needs to be learned is the National Electric Code,the edition that is adopted by your state. Some are still using the 2002, some are using the 2005 edition. The NEC is revised and republished every 3 years. Once a new edition is adopted, many states require those with licenses take an update course before their licenses is renewed.
So, start with the NEC adopted in your state, find out if and when they will adopt the next edition, if your learn the 2002, you will need to learn the changes in the 2005 of which there were many significant changes, that may only been simple wording changes for clarity to entirely re-written sections that are major changes in how something is done.
There are many books available to help learn the code, try first at
Mike Holt Enterprises for a sense of what is available. Read through this forum for real questions and answers to code interpretation and solutions.
A very good reference book that explains a great deal from theory, formulas, and real life equipment is the American Electrician's Handbook. Many of these books are a bit costly and must be considered as an investment towards the career.