Welcome @samzyannn!!
Is your project going to be inputting a line-level signal (like from hardware synths)? If so, you don’t really need to worry too much about additional circuitry for the audio input pins.
If you’re creating a guitar pedal, then you would need to amplify the instrument-level signal of a guitar which is quieter than the line-level signal. Referencing the schematic of guitar pedal project like this one would be a good start.
As for headphone amp output, we recommend checking out the schematic for the Daisy Pod.