First of all, you need to study science and mathematics courses starting from your high school. You should take geology as a major subject as it will help you to know all important things about volcanoes, and how they erupt.
Along with geology, earth sciences is also necessary for becoming a volcanologist. It will enable you to judge the variations of soil and anticipate the current condition of volcano.
While going to college, you need to continue studying geology along with Bachelor of Science which is necessary for becoming a volcanologist. If possible, try to study at a college which is situated near to those areas where volcanos erupt at regular intervals. It will help you to frequently observe the land and rock beds where the volcano erupts.
If your target is to become a part of a professional institute of geology, then you need to have advanced qualification like Masters of Science in Geology. However, along with advanced education, you should have practical experience as well. Try to find an internship with some professional volcanologist.
Make sure you avail opportunities of working with different volcanologists. You can also do an internship with some professional firm that deals with the study of volcanoes. It will help you to get a highly remarkable experience and will allow you to develop a good insight and understanding about te actual process.
If you have no problem like financial issues, then continue your study and complete your Ph.D. in volcanology which will make you a seasoned and qualified volcanologist.
After completing your degree(s), you should apply for a job in different governmental organizations like U.S. Geological Survey (USGS). You can also apply for a job as a volcanologist in the geological organizations within your state.