Hi Noah,
welcome to the forum

Nice first game... I guess there's a ton of stuff you could add to it

.. add sound, add some kind of power-ups, add explosions and/or particles when a bullet kills an alien... or just try to optimize the stuff you already have, add animations, add more types of bullets or different fire modes (more bullets per shot).
Personally I think the bullets are to slow and the alien movement goes too straight downwards. You could add some x movement as well or use some tweening or math.sin/cos to let the aliens fly kinda "wavy".
btw I think your code is pretty good for a beginner, often people (me as well) start without taking dt into account or stuff like that... You even have a readable code structure with proper code indentation... and comments!
When I started with lua and LÖVE, my code was horrible (I mean even worse then it is now

) ..
(oh and thanks for the proper download zorg. using ad.fly for these things is not a good idea :p and, measured by the people really downloading this, you don't get rich from it anyway.. so..)