This page was needs to be updates and/or broken up and/or made more useful/guided, probably pointing at categories!
So you want to make a mod for Stardew Valley. That's great! But where should you start?
First, ask yourself: do I know how to code? If the answer is yes, you can head right over to the official wiki and start poking around.
If you don't know how to code, that's okay! You can still make a Stardew Valley mod and it will be easier than you think. There are several mods that make it easy for anyone to add content to the game. The best known are Content Patcher and Json Assets. If you've previously installed any mods these were probably requirements for at least one.
Note: No matter what project you choose to start, you are probably going to discover that you missed a bracket, didn't add enough commas, capitalized something you shouldn't have, etc. This happens to every programmer ever. Fortunately, there's a tool to help you check your brackets and commas before you spend ages waiting for smapi to load: smapi.io/json. And keep in mind that that dopamine hit when you get everything working is going to be so great.