Getting ready to study biology? You may want to brush up on these foundational topics!

What should I know before starting biology?

If you are curious about biology or plan to study it in the future, you may be wondering what "prerequisites" it has – that is, what other knowledge will give you a solid foundation to learn biology. If so, big kudos for thinking ahead!
In my opinion, the only strict prereqs for biology are curiosity, an open mind, and a willingness to think critically about the natural world. If you have those, you can start learning biology without other background, as long as you're willing to pick up bits of chemistry, physics, statistics, and math along the way.
That said, you may find your journey through biology smoother and more satisfying if you already have some familiarity with topics in other areas, particularly chemistry. Below are some foundational topics that will help you get the most out of Khan Academy's biology materials (or any biology class).

General science skills

  • The scientific method. Are you rusty on what a hypothesis is or how it gets tested? How about experiments? These basic concepts will help you not only in biology, but also in any other area of science!

Chemistry

  • General chemistry. Get a feel for atoms, molecules, and how they interact with each other. After all, that's what you (and all life) are made up of!
  • Acid-base chemistry. A lot of the chemistry in your body is acid-base chemistry that takes place in watery solutions. Knowing what acids and bases are will get you a long way with biochemistry.

Physics

  • Laws of thermodynamics. Get a feeling for what energy is and what rules govern its transfer. Energy is constantly flowing through ecosystems, organisms, and cells, and is essential to keep these systems running!

Statistics

  • Basics of probability. Probability is a key concept in biology. You don't need to know tons of details or formulas, but if you understand the basic concepts, that will help you a lot with genetics and population genetics.
  • Statistics. Remind yourself about the basic ways we can describe sets of data, such as mean, median, and mode. If you go even deeper and learn about hypothesis testing, you'll definitely be ahead of the curve!

Math

  • Basic algebra and graphing. Most intro bio classes are not that math-intensive, but having an understanding of basic algebra and graphs (e.g., the meaning of slope) will help you understand figures and data in biology.

Do I have to know all these before starting?

Not necessarily! As I mentioned, you can also learn as you go. You just need to be willing to work on these topics in parallel with your learning of biology. So, don't be deterred from biology if you haven't yet mastered all of these topics.
Case in point: I was the poster child for how not to prepare for biology classes. I never took physics in high school, and didn't take it in college until I was a junior! I was also behind on my chemistry classes for most of undergrad. While I don't recommend that approach, it goes to show that a motivated person can be successful in biology even if s/he is "catching up" on some of the prereqs.

What if I don't like [chem/physics/stats/math]?

Don't be deterred from biology if some of these topics are not your favorites (yet!). Biology is a huge, diverse field. All biologists need to have some basic, foundational understanding of chemistry, physics, math, and statistics. But they don't have to become specialists in all these topics.
Also, if you had a bad experience with one of these topics in the past, why not give it a shot on Khan Academy? You may find it's more fun than you expect!