What is your typing speed, your spelling and grammar? Do you have polish? (If you have to ask, you probably don't). Many high quality executive assistants have at least a Bachelor's (may or not be required).
Aside from typing, you will need to schedule meetings, must have a great phone voice.
You can start out by looking for a clerical job, get one and then start building your skills. Start reading the Wall Street journal on a regular basis. Go to your county employment website and see what courses they have.