This is hard.
Until you figure out to use the controls immediately after starting, then there's an immediate exploit to never lose at all. A number of things for improvements:
-The artstyle is crude, but that's not a big issue for something that seems so early in development. The polishing comes later.
-You have no delays. After you hit start, put in another pause, with another "begin" action, and maybe a delay before the enemies come in. It lets the user get familiar with the game screen and be prepared for a battle.
-THE EXPLOIT: unlimited fire. All I had to do, once I actually knew what I was involved in, was to mash all three buttons. Spam the fire button, spam left and right, and now every entry point was bombarded with fire. Maybe add a fire limit, or a basic cooldown like a hot machine gun. Something that makes the player have to carefully choose when to shoot.
The programming, even if basic, seems to be correct. I didn't see any actual errors, and it ran smoothly, with how I assume you meant it to play. Great job there.
This seems like it would be a passable grade in a game development class and a great start to understanding building games, but still needs a lot of improvement for widespread enjoyment. Keep it up!