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You can go to the Microchip website and read some application notes.
But the best way to learn is to do something, like a small project.
Therefore, I would just go out and buy a PIC, say a popular PIC16F84A and use it in a socket on a proto board to build small projects. That is the best way to learn.
When you get stuck, post your question on this board and many members will provide an answer and guidance.