Hey @Sombrero06, and welcome to the discourse.
There are some threads here that give suggestions about how to start out with modding (a couple of them are Teach me to mod 😊 and Teach me how to SMOD), and they’re still relevant to this day. 
Some takeaways from those:
- Look within previously made mods. All mods are essentially .zip files, but renamed into .smod. As such, you could simply rename them back into .zip, extract their contents and see how they are made (this also includes the main mod which is stonehearth.smod, and they’re all located in Stonehearth’s mods/ folder).
- Knowing how to use Json (homepage) is important, but it would only take some minutes to read up on its syntax.
- For most modding ideas, you’d only need to use Json, but if you’re looking to add new features, then you’d also have to look into coding with Lua (in-game logic) and/or JavaScript/HTML (UI elements). For the ideas you mentioned, sticking with Json should be fine, but that also depends on what kind of job you want to add.
There are more points that could be made here, but I feel that those were the some of the biggest ones at the moment.
To add to the first point, you might want to look into mods that specifically adds content similar to what you have in mind (e.g., Archipelago Biome mod 🏝️ (better fishing and expanded campaign) which adds a fisher job, and a new biome, and look into rayyas_children.smod on how to add a new faction/tribe, if you were looking on adding a playable one that is).
Of course, if you have any specific modding questions, feel free to start a new thread with that question in mind (assuming no one has asked it before). Good luck!