Becoming a professional astronomer can take many years to accomplish and the job market in the field is competitive. However, it is very rewarding to be able to work at something you are passionate about. Many astronomers spend time in remote locations, looking at the sky through a telescope, but a lot of work is done teaching, doing administrative work and writing papers. There is always the chance of making a groundbreaking scientific discovery, and even having a celestial body named after you. Getting a job may be challenging enough, since demand for astronomy jobs is way higher than the number of jobs there are. Before that, you have to concentrate on studying hard.
Major in physics in college. You can do a double major of physics and astronomy, if it is offered by your school. It is also important to take a series math classes, including algebra, calculus and differential equations.
Get a PhD in astronomy or a related field. Search for place to do a PhD, including in the U.S., where it can take 5 to 6 years, or overseas, where it can be done in 3 to 4 years. Getting a PhD involves hard work, doing a lot of research, and writing papers and getting them published.
Apply for a post-doctoral research position. These can be named, such as prestigious fellowships, or unnamed. Either way, you will likely have the chance to work with important people in the field and research important scientific questions. You will have the chance to prove yourself in the field by writing publications, typically 4 or 5, maybe more, per year.
Find a research or assistant professor job. Network with people in your field, search job boards and make contacts in places you work. The more people you know, the better your chances of finding an open position at the right time.
Write your job application, get letters of recommendation, and prepare for the job interview. You may be required to give a seminar on your research as part of the job interview, so be prepared.
Continue researching, publishing papers, and finding outlets to talk about what you are doing, so that others in your field will recognize you and the work, and/or potential discoveries, that you are achieving.