Welcome to S-Tyrian
Hope you enjoy!
Down here is a basic how-to-play tutorial:

Move: Arrows or WASD
Fire: Space bar
Fire side-kicks: Z or M
Rear-weapon mode: Enter
Change sound/music state: Y

Sound samples provided courtesy of me and a microphone! Tapping 'Y' will change the sound effects and music (6 different settings).

There's a cheat code for infinite lives hidden in there somewhere, try your best to find it.

Also Have you found the six secret levels?

Good luck comrade :)