Very good begining !
I like a lot this 3D retro style, reminds me Dot Game Heroe on PS3

Some ideas :
- Add a little inertia to the ship ,and make it move smooth !
- Add objects in background like planets, flying ships,
meterorites, space workers on a station working or a simple
skybox
- Later : better ennemi explosion effects : perhaps explosion in some cubes flying away.

Good job for a first game, bring on some big bosses later !

Keep up improving it