No idea where to start
I am trying to create a program that when run will open a window with a form that the user fills out. When the submit button is clicked, the user input from the form will need to be saved into a text file. I have no idea where to start, or even what I should google to ask for help, so any info is helpful.
You have 2 distinct parts: GUI and writing to a file. Find a Swing tutorial for the GUI and an IO tutorial for writing to a file.
Hint: google "really big index" and find the sections on what junky mentioned.