What steps should I take in college to become an Astronomer after college?

It is very hard to pick between sciences when you love them all. Thankfully these days it is becoming easier and easier to work in a space that is more than one field of specialty. The days of "I am a physicist," vs "I am a biologist" are quickly fading. Many physicists work in biology and many biologists need a great deal of physics to perform experiments in biology. So my first piece of advice is don’t worry so much about putting yourself in a specific title so soon. Study both physics and biology as you go. You may find there is a specific area of study that you enjoy that skirts both sides of the fence and lets you enjoy all that both fields have to offer.
Take intro classes in both subjects if you have not already, and then keep going for at least the second year into the sophomore level classes. Only then should you even begin to think about picking. After all, learning is always valuable, even if it doesn’t get used directly in our daily jobs!