Speed.cd
Came across a document provided in a torrent that had something along the lines of "Downloaded from Speed.cd" and I was like "Hey, Imma try it out." So, I came to the site and saw that it required a membership and thus I was like "Huh??". At one point there was a "Try your luck and tell us why you would like to become a member." form. They also notated at the bottom "You only get one chance. Don't waste it." or something along these lines. I gave it an attempt and was approved. And so then began my journey of Private Trackers. Of course, it was TI that shed light on what a Private Tracker even is. Love TI. <3

SCC
Too bad i lost the account :/

I think it was Demonoid if you count that as a private tracker. Still have an account there that is over a decade old.

My very first tracker was IP torrents and still has good content!!

It was HD-Torrents, a very good HD tracker with excellent internal groups!

My first private tracker was torrentbits.

backups-me was mine, it wasn't the best but ok for my first ever tracker i joined

ooh takes me back :3 I think the absolutely first was one of these two
Powerbits
Piratebits
Then I had TTi which I'm preeeetty sure are todays TI (TheInternationals) but changed name and I think the staff might be diffrent.
But the theme on the website is like identical

Mine was t411. I think the most complete french community torrent seeding. Inscription still open

My first private tracker was SceneTorrents 10 years ago. Used it until they closed