You can take a look at
https://wiki.alliedmods.net/Introduction_to_sourcepawn . Note that this is pre-1.7 syntax, but it is still valid. It is also closer to amxx syntax which you (indirectly) said you know.
Also
https://sm.alliedmods.net/api/ and
https://sm.alliedmods.net/new-api/ (I find the older better and with user comments, though they started deprecating it
, so the new-api has additional content ).
I personally learned mostly by looking at the code of the already existing plugins, and manually searched key words in the online api tons of time.
https://wiki.alliedmods.net/Game_Events_(Source) is also very useful.
Environment-wise, you do not need any special tool. A text editor does the job. Notepad++, Sublime Text and especially
SPEdit recently seems to be liked a lot. And of course you may want to read the stickies. One does mention the compiler for instance.
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