Technically, the first computer I used was a General Electric 635 mainframe. (You may not have known that General Electric ever made computers; GE sold its computer line to Honeywell long ago.) I didn't use the 635 much at all, but since we computer people are sticklers for accuracy, we've got to count it.
My interest in computing came courtesy of Richard W. Ramette, a chemistry professor at Carleton College in Minnesota. I corresponded with him while doing a science fair project as a sophomore at Detroit Catholic Central High School in 1972. My chemistry teacher, Rev. Leo J. Klostermann, was at the time the nation's leading producer of International Science Fair winners, and I was in his "farm system". Unluckily for Fr. Klostermann (perhaps), I eventually found the computing more interesting than the chemistry.
Ramette sent me the IBM 7090 (!) programs he used to crunch spectrophotometric measurements to calculate equilibrium constants. But the stuff I was doing was much simpler, and I didn't need the full power and complexity of his programs. Plus, I didn't have access to a 7090. As a matter of fact, the calculations I needed to do were so simple that I could easily have done them all for the entire project with a slide rule in an hour. However, I wanted to pursue the computer angle.
My father got a colleague of his at Ford Motor Company to help me. Leo Maher had an interesting background. His name was an unusual one for a Chinaman; an orphan, he was given the name by the Irish missionary who raised him. Maher not only knew several programming languages, but he spoke five natural languages as well.
Maher quite reasonably figured he should start me out on BASIC. He wrote one or two very simple programs to my specifications, and then, using them as a model, I wrote a few more. Mostly, I wrote them on paper and sent them to Maher via my father. I believe I did get to use the Teletype in person once, though.
My programs, because they were very simple and were based on Maher's clear examples, did work. I didn't really know what was going on, though. I don't think I even knew that I was writing in a programming language named "BASIC".
Before I got too deeply into the 635, I switched to using my high school's Honeywell 1640.
One last note on the 635: this computer ran an operating system named GECOS. The name of this OS still lives today as the name of one of the fields in a UNIX password file. In the early days of UNIX at AT&T, the GECOS field had information need for the UNIX machine to interact with AT&T's Honeywell (or General Electric) mainframe.