There is a lot of online resources for Stardew mods, the wiki has all the SMAPI docs on them
Creating SMAPI mods
stardewvalleywiki.com
This is the best place to start, start small and get something super basic working. Then try what you want and make small iterations between lots of testing.
Software I use Visual Studio Code
For art I use Photoshop
I am very new to modding in Stardew, I mostly just fix bugs in the mods i use already.