G. Mark’s Answer
You need a desire to be a good mechanical engineer. That's it. Simple. If you really are this type of person, you'll have been attracted to things mechanical all your life. In my case, I was a toddler who wondered how the weird cat clock on the wall made its tail move back and forth. So I got a little pocket screwdriver my grandfather had left in a drawer and pulled out the drawers in the chest below it like steps and got the thing and took it apart and then put it back together. I'd say a lot of mechanical folks started out taking apart something they shouldn't because they were compelled to. After that, you'll probably have a life of poring over magazines with pictures of the insides of machines and watching cars and planes and rockets and vacuum cleaners with more interest than most people. And then you'll end up in college or a trade school. Then you'll get a job of some kind doing what you like to do. And you'll end up a good mechanical engineer or service tech or designer or a person who fixes stuff. And you'll be good at it.