Tech diving, equipment, awareness and too much too soon
This thread is a good read and a good start. There are many others here as well.
Most important is to find a great instructor - the agency isn't really important although some will disagree with me there. You need an instructor that you will be able to work with. There is no comparison to OW training and tech training.
If you haven't dove doubles and a dry suit. Start there, deep diving is going to require that you do both.
As for the first class, advanced nitrox and deco procedures is a good place after you have gotten the skills down on doubles and drysuit diving.
Be prepared to be in debt for the rest of your life. Tech requires lots of equipment - my garage could double for a rather large scuba shop
Cheers
Steve