Getting Started - this is a good place to start!
FAQ - Frequently Asked Questions - see if your question has already been answered.
Development - information about PPSSPP and PSP development.
Reference - specific documentation about various features of the emulator.
If you can't find what you're looking for above, use the menu to the left (or the "three lines" or "hamburger" menu in the top right if you're on mobile) to browse the documentation.