Anyone know a reliable site where I can learn Electrical, Computer or even Mechanical Engineering

Am a high-school student , and am undecided on what I want to major in. Am trying to self-learn on my own so by the time I get to college I'll know what I want to major in.

For electrical, search for Lessons in electric circuits by Kuphaldt in "discover circuits"

Here's my recommendation. Look on any University's course catalog for the classes required to get a degree in Mechanical, Electrical, or Computer Engineering. Then, look those subjects up on Wikipedia. For reference I will include links for U of I and, of course, Wikipedia as the source.

A good practical minded approach would be to work with a pic processor card called the Arduino.
It would be a lab style approach; better than just reading. There are plenty of books and videos.
Another suggestion would be the youtube website, a channel called Afrotechmods.

Yes. MIT has a web site that you can use to learn any topic they teach. Just don't expect to get credit for any of it unless you pay for the course.