Discounting my BASIC bits and pieces written on a Vic-20 and an Amstrad 464, my first ‘heavy-duty’ program was probably a college assignment coded in Turbo Pascal or GW-Basic. Might have even been C-code written in a mainframe environment (PDP-11 or VAX). In any case, it would have been a business-related processing system or some sort of ‘algorithmic challenge’ (eg Fibonacci numbers, etc).