I'm not very experienced, but I think I can help.
1. Look out for free sfx to add (or for someone who can make them). For sprites it's the same: free assets or team up.
- There's a lot of free stuff out there! (Remember to credit everything. You could keep a file just to keep track of that)
2. I don't know how exactly your game is, but you could try making a levels system. Your first level could be focused on introducing the player to your core mechanic and the enemy
- Your game doesn't need "levels", it's just an example. You could call it other things, like "areas" or "challenges" or add another way to make progress.
3. After this level is done, you can expand on things. You can create new enemies and/or mechanics. Always remember to first introduce the mechanic/enemy before using it in more creative ways.
4. Add juice and nice touches. You can make things smoother, add a main menu, mid game menu, volume options.
Also, it's normal to a game start that way: just a simple mechanic with bad art and nothing else. It's your base for the rest of the project.
I've already volunteered for another project (they didn't accept me yet), but I could try making some sprites for your game if you want.