Before you buy the banknote, buy the book.
The local library is a good start before you buy the book.

I recommend collecting star notes, more rare.
If you are getting notes from the bank or from circulation, go for CU (cut well centered and uncirculated) first.

I use to get my star notes from my wife, she was a casino cashier.
When they get new bundles she searched them and hold the stars for me.
You could ask your favorite bank teller or store cashier to be on the look out.

Good hunting.