Step 1: Decide on what type of currency to collect
Large-size types, lower denominations: A collection of $1, $2 and $5 values.
Interesting serial numbers: Low serial numbers such as 00000001, 00001429, 000016776, and others sell for a premium. Another example is a "radar" note, which is currency with thew same serial number forward and backward, such as 12344321 or 44888844.
Star notes: When a note is found defective after printing, it is replaced with a star note. On federal reserve notes, the star is after the serial number, while on all other issues, the star comes before.
Currency errors: All sorts of interesting errors and misprints exist, such as a double denomination, such as a $10 printed on one side and a $5 written on the other. (The serial numbers were blurred a bit.)