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SA-MP gamemodes are (usually) scripted using Pawn.

All the documentation about how to use Pawn for SA-MP-specific purposes can be found on the wiki: https://sampwiki.blast.hk/

Would you want more info about Pawn itself, you can check out the Pawn Language Guide: http://www.compuphase.com/pawn/Pawn_Language_Guide.pdf