Start reading the datasheet. It is actually organized in a manner such that you learn the basic parts of the chip first, and it goes into detail about the 'extras' later (with the exception of the instructions).
You first need to learn the memory organization.
Then read about how the memory, CPU, and the data bus work.
Then read about the Ports.
Then read the instruction set.
After that much, you have enough understanding to do simple projects/codes.
After understanding those basics, you can then move into:
Interupts
ADC
USART
TIMERS
Comparators