I cant understand mechanical drawing,but Im decide to design engineer?

design engineering requires multiple disciplines. including Mechanical, electrical, and civil engineering. A design engineer might also work with Industrial Designers.

Your tasks as a design engineer also requires skills in autoCAD such as Autodesk Inventor. other tasks include prototyping and knowing Virtual Prototyping software like Ansys or Comsol. You will also be working with Manufacturing Engineers, Quality Engineers and Statistical Process Control and I'm sure there's others.

There is a reason why you hear stories of sleepless nights. I've only just begun my journey. I'm not going to school for it yet but I'm learning a lot of the basics on my own. Recently I've caught the flying bug, so I'm thinking of becoming a commercial pilot instead.

But don't let these requirements discourage you from pursuing your dreams. take things one step at a time. Don't take more than a couple of classes at a time because it requires a bunch of homework. If you have any other questions on this I will be happy to help answer them for you.