Is there any mechanical engineers? What you would wish to know when you were starting learning? What skills and topics you consider the most useful in mechanical engineering?
What is the fastest way to learn mechanical engineering in the nowadays when 3d printers are avaliable to be able to design and make custom clocks, engines, generators?
3D printers aren't really going to help. Mechanical engineering isn't about making things (although that's certainly one outcome), you need to know the basics about materials, free body diagrams, how structures behave… it's a bunch of pockets of knowledge that combine to give you the basis of your understanding. From there you decide what to do with it - design things, analyze, combine this with other areas of knowledge, the possibilities are quite endless.
But there is no "fast". Unless you are quite literally a prodigy with a photographic memory and can understand and apply concepts quickly, there's no shortcut to getting there fast. In fact I'd suggest you want to do the opposite, pick an engineering school with a strong co-op program, and both learn and apply what you learn.
Indeed. It took many, many years of experimenting and building personal projects before I even got to college to build the repertoire of understanding that goes into building designs that are functional, safe and enjoyable to use.
The sort of dark humor joke we pass around (of many) is, "anyone can build a bridge, only engineers can build one that barely avoids collapse". But think about what that means: it's a bridge which stays standing in all expected weather and geological conditions, it is intuitive to use and maintain, and it is efficient in material usage.
Nothing to it but to do it: work on personal projects, pick a new topic and learn about it, while making experiments to demonstrate how your new knowledge works to yourself.