I completely understand. Sometimes narrowing down your thoughts into a few cohesive thoughts can be a real pain. I don't know if your interests are in all of biology, or if you are hoping to focus on macrobiology or microbiology. Perhaps you haven't thought about it with that kind of specificity. It might help to do that now, because it might alert you to your end goals.
I've always liked macrobiology a lot more. That would be studying animal behavior and ecosystem stuff. I've always liked being outside and so focusing my efforts there have allowed me several opportunities to do some cool field work. I'm just not into staring through microscopes all day.
Maybe your interest in biology stems from a desire to better understand how the human body is put together. That would put you on track for something like medical school. Veterinary school might appeal to you too.
For me, I like studying and teaching biology because it explains how and why living things work. It's mechanisms are visible, and they are concrete. Chemistry is okay, but there is too much that is so small. I also like knowing that understanding biology is immediately applicable. Whether it is how to better care for the planet, injury maintenance, healthy dieting, etc., biology just seems to affect so much that I'm passionate about.
For your statement, maybe the best way to start is to think about who instilled an absolute love of science in you. Then focus on why biology is your passion over other sciences. From there, write about what you would like to do with your biology education.