Like the others said you need to research and find out what YOU want to look for. There are a few dozen major varieties that everyone hunts and then there are countless minor varieties that you'll have to decide if you are going after.
Flip around all of the sites that John listed do a search in this forum, people have posted their list that they look for. If you think you have something different then coppercoins is the best place to look them up.
What coppercoins said above is the best advice though, to find varieties or even errors you have to have a basic understanding of what normal is. That takes a lot of time and practice but most people are here to help so post coins when you think you have something.
Die Deterioration and
Machine Doubling (MD) are very common struggles for people so search for those topics in the forum and read up.