You will start your career as a payload specialist. Payload specialists are the additional crew members on the spacecraft. You will help astronauts carry all the essential equipment during the journey. You will also check the equipment from time to time and inform the pilot about any glitches.
You will then become a trainee astronaut, where you will assist other members. You will help them in all their tasks. For example, if there is a technical problem in the spacecraft, you may have to locate the problem and inform the working astronaut.
After gaining experience, you will move on to becoming a junior astronaut. Here, you will pilot the spacecraft so the senior pilot can take breaks. You will also fix technical problems on the spaceship, if any.
Top astronaut pilots or commander pilot astronauts are responsible for both the spacecraft and the crew. You will fly the shuttle and communicate with those on Earth through satellites and other communication devices used while in space.
A mission specialist leads a team on a particular mission. For example, if you are sent to Mars on a mission as a mission specialist, you will plan and execute the entire space journey. You will also plan activities for the crew, usage of consumables like food, water, etc.