Hi Ted,
Welcome to the forum. Processing has it’s own way of handling mouse and keys. Processing reference https://processing.org/reference/ is a good source to find these things. I know you don’t always know where to look at, so you can always ask the forum. Anyways processing has several event handlers for mouse. Event handler is a function that is called by the processing when something happens with the mouse. The one you are looking for is mousePressed() https://processing.org/reference/mousePressed_.html. If you just need mouse coordinates inside the processing window system variables mouseX and mouseY contain them.
Turorial https://processing.org/tutorials/overview/ explains this all too.