Astrobiology is a vast science covering everything from biology, geology, astrophysics to engineering. Therefore, you could study a number of space-related sciences in order to be able to become an astrobiologist one day. It really depends on what area of astrobiology interests you the most. I personally was really interested in studying extreme life (extremophiles) on Earth as analogues for life on other planets. That is why I ended up first studying planetary science at UCL (a combination of geology and astrophysics), but then I specialised in microbiology and environmental science to get into the field of extremophiles. Having this multidisciplinary education is very valuable in astrobiology and I highly recommend it.