Hi! That's a great question! Choosing a particular field of engineering can be a really difficult choice. Many colleges admit undergrads into "general engineering" when they first start college. You'll probably take many of your general ed courses during your freshman/sophomore years. Then, you'll be asked to "matriculate", or choose a field of engineering, somewhere around your junior year. The type of engineering you choose at matriculation will determine which courses you take for the rest of your undergraduate program of study. Some fields are very closely related, so the coursework will be similar. Switching fields may simply require taking a few extra courses to cover the requirements for the other field.
I'm not sure about graduate school requirements, but once you begin interviewing for jobs, employers look at your transcript as well as your willingness to learn new things. I received my BS in Mechanical Engineering, but took a job right out of college as a project engineer, doing lots of Electrical Engineering tasks. I told my employer that I was smart and eager to learn new things, so he hired me! As a result, I learned a lot of electrical engineering skills on the job that I never learned in school.
My advice: choose a field that interests you, then see where it leads! Good luck!