Yeah FreeSound is great and there are graphics and sounds on Open Game Art:
https://opengameart.org/content/library-of-game-sounds
In the future, if you are getting more into it and want LOTS of sounds/music, SoundRangers has ones you can buy.
If your game is realistic, you can make it with your phone and your voice!
If it isn't, then you could use a sound/music editor that has built-in instruments. (Pico-8 has one for more complex sounds, and can export WAV files.) Here are a bunch of free ones:
https://www.techradar.com/news/the-best-free-audio-editor
Good luck!