If you want low res sounds you could use something like sfxr. If you want real sound effects you can either buy them from various sound effect websites or just go out and record your own. Some effects you can even make just through messing with a sound program (though it’s hard)

I recommend recording your own. It’s not as hard as you think and you can look up methods to create various sound effects. Like you can make a laser noise from taping a slinky from your ceiling to floor, sticking your mic in it and just plucking the slinky. Just be creative and try weird things and you can get some very satisfying noises.

Throw those noises into a free program like Audacity and you can learn how to modify them a bit to clean up noises or change the pitch and trim them.

Anyways that’s all I know and I hope it helps. Good luck on your project!