Perhaps, first a question as to what sort of script you're looking to write. If you're looking to write a script using the ash language, then Notepad should be fine as your text editor and saving it with a .ash extension should also be fine. You'll want to save it into your .kolmafia/scripts folder so that mafia can access it. On the other hand, if you're looking to write a CCS (Custom Combat Script), you can do so right in your mafia interface. Have you checked the Wiki linked at the top of this page? It has quite a bit of documentation on functions, syntax, etc. You may also wish to do some searching in our repository, it's possible that what you're looking for may already exist in a script that you can simply use.
Welcome to the forum!