That is as wide a question as "what is the answer to life, the universe and everything?"
The basics:
You need to understand the behaviour of the basic electrical components, resistors, capacitors and inductors.
How those components behave when voltages are applied to them and currents flow through them.
Then you can move on to the active components with more complexity, diodes, transistors etc, leading on to integrated circuits.
Then understand how all these components can be connected together to make simple circuits having one function.
Then understand how to connect the simple circuits (building blocks) together to make more complex functions.
To achieve this you will need an understanding of some mathematics if you want to be able to calculate the required values for the various components in the circuits.
JimB