That is a very open-ended question. Mechanical engineering is probably the most diverse of all the engineering disciplines. I have designed machines my whole life. But I have worked with mechanical engineers that designed HVAC systems and wouldn't know how to design a machine if they had to. There are others that design piping systems, or structural framing systems, or bridges, or aircraft, or weapons, or biomedical devices, or ... There are others that just write specs for turbine engines or consult as trial experts or ...
What do you want to do?