Sunshine 1298,
The first step for any collector is to learn about the items being collected. For coins, the first stop I would recommend is the
RedBook. Inside you will find essential information about
US coins and the minting process, some common errors found in a series, etc. For your particular coin, you will find that all US circulating coinage from 1965-1967 were minted without mint marks (an interesting story in itself). In addition, coins minted in Philadelphia had
no mint mark and were often referred to as "plain" until 1980, and for the cent, much later. Why? How did the branch mints apply their mint mark?
The reasons and results have led to some interesting varieties. This is a very interesting hobby to learn about.