My first "finished" game was a 2D non scrolling shooter. (1 screen per level) where you used what was supposed to be a WW1 aircraft to shoot down other WW1 aircrafts.
The aircrafts were 2 rectangles, one big and a smaller one on top, + a line in front for the propeller, there was a green rectangle for the ground and a light blue background with a yellow circle in a corner (the sky and the sun), i made it in Pascal when i was 11, the levels were hardcoded, 3 levels(each with its own block of game logic, rendering code, etc), 1 enemy on lvl 1, 2 on lvl 2 and 3 on lvl 3), there were no functions (just labels and goto), all variables were global, the "AI" just had the enemies move from left to right and try to match your altitude. i even printed "nice looking" labels for the floppies so i could give copies of the game to friends. (This was before this thing called the internet became popular)
After that i read denthors VGA tutorials, learned some x86 assembly (i got a copy of those on a floppy somewhere still) and made a really awesome snake clone with a proper game loop, highscores, and some pretty nifty palette effects.