How can I start learning about electrical and computer engineering?

I suggest you try to find a local chapter of IEEE and ask if you can attend some their meetings as a high school student. Talk to the members and ask a lot of questions.
While in high school, you need to really learn all the math and physics that you can. Take the AP classes or even junior college classes in those subjects if you can.
Lastly, you can buy electrical and computer engineering handbooks to read. They are readily available.