To edit an existing script, you double click the script from within the editor. It will be in the Assets folder (or a subfolder usually, unless you like insanity). This will open your code editor and the script. By default this would be visual studio unless you chose to not install it with Unity or configured Unity to use another code editor.
To create a new script, you right click under the Assets folder and create the new script there. It will use a template to create a pretty minimal MonoBehaviour class with the same name as the filename you chose. This is a good starting point in most cases. The MonoBehaviour class and the filename need to be identical by the way, so if you change one you need to change the other or Unity has a panic attack 
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Off topic, but I also created this other thread for folks like you. Good luck and have fun!