I'm going to be getting a new PC soon, how do I switch my account to my new PC and make it unavailable on my old one?
Download and install the steam client on your new PC. You can now download and play your library of steam games
You can uninstall steam and games from your old computer's hard drive but if you aren't selling it/giving it away/throwing it away you can leave it there.
If you didn't set an email account as a recovery option then you will have to submit a ticket through the Steam website and explain the situation. They will likely want you to send them an invoice of a game you bought as proof of ownership.