Simple shapes for objects. Search for open source textures for things like rock, wood, tile, etc. if you need them. Make sure to attribute what you use to the authors.
https://jummb.us/ to make a simple chiptune that loops for your music. Use your webcam or laptop mic to bang on pots, pans, your wallet, books, etc. for impact or collision noises. Record very serious sound effects that you make with your mouth for odd needs.
Focus on something that is always fun and always getting better. A fun core mechanic. Add new enemies, weapons, "coins", etc. as you can.
Consider a simple menu and simple end screen with a play again button early to make testing easier.
Upload your game to Itch.io asap to ensure you know how to get it up, your game size is good, and it looks good online.
Add some more details on things you need and folks may be able to chip in more on how to get things done.
Good luck!