What kinds of plans do you must plan ahead when becoming an engineer?
Graduating high school with just a couple AP tests won't make you an engineer; you usually need to go through university to do that. First you have to find out what type of engineering you're interested in.
Do you want to become a civil engineer? Electrical? Software? Biomedical? Mechanical? There are many different disciplines of engineering.
Most engineering fields, however, do require a heavy mathematical background, as well as some division of physics or chemistry. For example, an electrical, computer, or software engineer would need to know about circuitry, electricity, magnetism, etc. However, a mechanical engineer would be more inclined to know about thermodynamics, stress/flows, liquid volumetric flow rate, etc.