start with the datasheet
. it's linked from the product page.
pin nine is power and pin ten is ground.
pins 15,16 and 17 are address pins. just them to ground.. if you are going to put multiple mcp23017's on the same bus you can connect some of these pins to 5v to form up to seven additional addresses.
pins 12 and 13 are your i2c bus. 12 is the clock line and 13 is data. you might want pullups on these lines, though the arduino is supposed to take care of that.
pins 21-28 will be referred to as pins 0-7 in your code. pins 1-8 on the chip will be pins 8-15 in your code.
aaaargh. what am i doing? all of this is explained in the example code
in the library.
"If I had known it was harmless, I would have killed it myself." - Phillip K. Dick, A Scanner Darkly