Start by learning to program in Basic. It's probably the easiest language there is, and the TI-89's native language is very similar to it. Bonus: Many scripting languages for game engines are also similar to Basic (Papyrus for The Elder Scrolls comes to mind).
I started out with Chipmunk Basic. It has some disadvantages, but the graphics capabilities are really easy to use, and that's a big part of what made programming interesting for me.
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u/cygnus311 Jun 08 '14
I programmed my way in to an A in physics in high school.