Alternatively, before delving into Allegro, you can start by looking at the code of some rogue-like or MUD games to get a better grasp of game programming in C using a simpler graphic interface like PDCurses. Not being too cluttered by a more complex video and multimedia programming library, these source codes better illustrate a game programming structure with its main loop and how to branch out from there into the several processing tasks a game demands. Rogue-likes will give you a glimpse into single-threaded game programming (most of them anyways. Some are multi-threaded) and MUDs will help you better understand multi-threading and network game programming.
You should then be able to better understand how to incorporate Allegro into game programming, once you learn about the usual structure of a game written in a procedural programming language like C.