This is not too bad for your first game, however there was one problem I had with it. It is either incomplete or the game has a coding glitch preventing you from finishing it at all. Would not mind seeing a second version of this game actually, seems like you are off to a good start with making this game too.
The graphics were great actually and the strobe floors, platforms, and ceiling for the first couple of levels is pretty nice. Then the floating blocks make me think of the first level from Super Mario Bros. 3, same thing with the water that is seen too. Then the last level is similar to the first with the strobe platforms changing colors. The enemies also looked decent as well.
The game play seemed simple and jumping was easy too. Enemies did not take long to beat and it seemed there was only one boss in this near the end of the second level you go to which did not take long to beat. The only enemies that seemed completely unbeatable were the Koopas since I could not seem to destroy them, just push them back a bit.
The major lacking point in this game is the audio. Sure, you got sounds for firing bullets but that appears to be it. No music, no finishing level sounds or anything else. Feels more like a game in beta form at this time. Adding music could liven this game up quite a bit.
Overall, not a bad start to a game, the only major things missing are music and the game being completable. Currently not completable since the last level going to the last platform takes you nowhere and jumping beyond that there appears to be no platforms to reach for.