Get comfortable with the big idea of differential calculus, the derivative. The derivative of a function has many different interpretations and they are all very useful when dealing with differential calculus problems. This topic covers all of those interpretations, including the formal definition of the derivative and the notion of differentiable functions.
One way of thinking about the derivative is as instantaneous rate of change. This is quite incredible because rate of change is usually found over a period of time, and not at an instant. Get comfortable with this approach here.
There are two ways to define the derivative of function f at point x=a. The formal definition is the limit of [f(a+h)-f(x)]/h as h approaches 0, and the alternative definition is the limit of [f(x)-f(a)]/(x-a) as x approaches a. Make introduction with these two definitions.
The derivative of a function isn't necessarily defined at every point. Learn about the conditions for the derivative to exist, and specifically about how continuity fits with this story (spoiler: for a function to be differentiable at a point it must be continuous at that point, but the other way isn't necessary).
This may blow your mind, but the derivative of a function is a function in itself! Get comfortable in thinking about the derivative as a function that is separate from, but tightly related to, its original function.