Best way to read inputs?

I am very new to this so please excuse the simplicity of the question. I want to read an input from a sensor on an analog input pin to trigger an output. I have heard that I should save the input to memory and read the memory to make a decision about the output. Is this what the serial monitor does or should I create a variable and assign it to that input and then do a Boolean comparison to that variable for the output decision?

You are mixing a few different things.

Start with the examples.
Try to blink a led. And send a message to the serial monitor. Toggle the output. Read an analog input and send it to the serial monitor. And finally do everything and you have your program.

But make very small steps. The first step is to load the blinking led example and make the led blink.

The serial monitor is simply a passive presentation medium; it cannot make decisions.