I know my username and my password, but I have no idea what my Player ID is, and I can't find it anywhere. Help?
Your Player ID is a number you receive when you participate in real-life tournaments for the Trading Card Game or Video Game. It's a part of the program called "Play!Pokémon" (formerly "Pokémon Organized Play", or "POP"), and the ID is also referred to as "POP ID", "Play! Pokémon ID" or "P!P ID".
Anyone who has one of these can connect it to their Trainer Club account in order to track their achievements in Leagues and tournaments as well as go through certifications as part of the Professor Program.
If you've never participated in a sanctioned official tournament or League, then you don't have a Player ID. If you've never registered it to your Trainer Club account, you don't have one on your account. Since using one is optional, you don't need it unless you want to play in official tournaments or Leagues (in which case you'll get one on site) or certify as a Judge or Organizer (in which case you should find a League near you and go there at least once to see how things are done first, and you'll get an ID there). If you have one, but never registered it on your Trainer Club Account, you can do so somewhere in your Profile (I don't know exactly where since the option disappears once it's registered).