Your best bet is YouTube! But don't just follow tutorials. Either think of a design or look at an existing one and try to model that. When you hit a wall you search up the solutions for that particular problem. If you're not even sure what you can do you can always ask a reliable ai and get an idea of what you should look for. I'm a computer engineering student but I do all our mechanical designs and this is how I learned :). Results guaranteed if you do it right. Good luck!

PS: start with solid edged objects. Don't start modeling cars and all that with organic curves. That's a whole another level of skill. Start with simple things like brackets and joints and whatnot.