I recently started looking into modding for the very first time myself - based on my progress despite my limited experience prior this, I’d say Stonehearth is a good place to start.
(Just in case you’re really curious, my first attempt at modding is found here)
Modding in Stonehearth is performed in JSON-files when it comes to static things like adding a new recipe or a new workbench; things that requires logic is performed in Lua. (Ergo, most of the time you’ll work with JSON)
I’ve never worked with either of these prior creating my first Stonehearth mod - yet I find it somewhat straight forward today. If you have any prior coding experience at all, I’m sure you will find it “easy enough” to be able to make good progress on your own.
And, if you ever get stuck, it seems to me there are some high-skilled, helpful people on these forums 
Adding Models
Do you want to create your own models? If so, you will need some modeling software that can export .qb-files.
Cubicle is what the devs use; it is pay-to-get. MagicaVoxel is what I use; it is 100% free. (These are just two examples - there are more softwares out there)
Basic Modding
When it comes to the act of modding itself, I would suggest you to download the official starter-mod and learn as much as possible from it:
It showcase how to add custom recipes and new entities into the game.
In order to find more examples or even look into more advance stuff, you can learn from what already exist within your copy of Stonehearth:
- Within \steamapps\common\Stonehearth\mods, create a copy of stonehearth.smod.
- Rename the copy to stonehearth.zip and unzip it - this will now yeild a folder namned stonehearth.
- You can freely browse this folder and everything found within regular Stonehearth - it makes an excellent way of learning in my opinion.
Getting Help
As previously stated, there are many talented modders on these forums - browse these categories and surely you will find some help.
https://discourse.stonehearth.net/c/modeling-animation
https://discourse.stonehearth.net/c/modding
If your question is not already out there, posting it on the forums is bound to yeild some help, too
Coding Software
In order to work with JSON and Lua you will need a text-editing software. I’m sure there are many fancy software out there, but even WordPad will do, really.
Personally, I’m using Notepad++.
Using “my” method, it is not too easy to validate the JSON-files all the time.
So, here’s a link to an online JSON validator, too.