I had a TRS80 Model I, too! My first program was written in BASIC on a TRS-80 Model I, but I ultimately learned to program in BASIC on the Tandy CoCo 2, using the Color BASIC and Extended Color BASIC books.
In retrospect, it was a somewhat harrowing way to write any kind of code. We didn't have IDEs. We typed our lines of code straight into the command line, and if we wanted to read the lines we had already written, we had to dump the range we wanted to see to the screen. There was nothing like code completion. We didn't have anywhere to look things up except for whatever books we had on hand. There wasn't any Google, or StackOverflow, or anything like that. Nobody else knew how to program, so there was nobody to ask for help. There weren't any standard libraries for anything... anything at all. My programs had to be saved on cassette tapes. You had to really want to program, but it was such a power trip!
In my teens, I learned my second language, Pascal.
(I went on to use 18 other languages, after that!)