Unity is its own beast, you should look into getting used to the unity interface if thats the path you want to start down.
An IDE is the program you use to write your programs. (integrated development environment)
Start with small text based c# console applications, but realistically if you are starting from ground zero, there are a ton of c# just getting started tutorials online that would also be very helpful.
To start look for a tutorial on how to setup your IDE with a new project and how to compile that project.
Then move on to figuring out how to make the program output text to the screen.
If you can get that far the next step will be trying to get input from the user.