I have a question about resistance in circuits.

I understand Ohms law and I understand how to calculate resistance in an existing circuit. My question is, how do you determine the required resistance in a circuit that you are creating?

Say I have a simple circuit with 1 LED. Because of Arduino projects, I know that it will require a 220 ohm resistor. However, this may not always be the case. As I continue to grow and build more complicated circuits, how am I to determine the required resistance in order to select the correct resistors to use? Is it in the tech specs of the components? Or is there a way for me to measure and test my circuits without the resistors in place in order to use Ohms Law to calculate my needed resistance?

I very well could be overthinking the heck out of this one.