I mean, for a first time it's not bad.
If you want my feedback, here's what i'd improve:
-some audio would be needed. You can add an "Audio Source" to a GameObject so that it can play an audio clip. Check this video from Brackeys
-the enemy AI just tries to get close to you, witch is the simplest thing you can do, but it doesn't look too good when even the boss gets stuck and you can shoot him from the distance. I'd suggest, from easiest to more difficult, placing the layout of the level better, making the enemies shoot you and adding some path-finding algorithm (check Astar if you're curious)
-the button to return to the main menu at the end of the game doesn't work very well, try checking it's boundaries
This said, you're just ad the beginning, keep improving and you will be able to create wonders