I have seen on Netflix that there's "a certain magical index" and "a certain scientific railgun" my question is which one should see first so which is "the first season"
The easiest way is to look at it in the order of publication.
Index 1
Railgun 1
Index 2
Index Movie
Railgun S (season 2)
Index 3
Accelerator 1 (currently running)
Railgun 3 (will be released)
Here are the release years of the respective seasons. You can look it up in the publishing order to understand it.
Toaru Majutsu no Index (2008)
Toaru Kagaku no Railgun (2009)
Toaru Majutsu no Index II (2010)
Gekijouban Toaru Majutsu no Index: Endymion no Kiseki (film, 2013)
Toaru Kagaku no Railgun S (2013)
Toaru Majutsu no Index III (2018)
Toaru Kagaku no Accelerator (2019, currently running)
Toaru Kagaku no Railgun 3 (expected to be available in 2019)
Otherwise, you could also watch it in chronological order, but that's awkward because you have to jump between your individual seasons.
Railgun 1-14 → Railgun OVA → Index 1-6 → Railgun 15-16 → Index 7-9 → Railgun 17-24 → Railgun S 1-10 → Index 10 → Railgun S 11-16 → Index 11-14 → Railgun S 17-20 → Index 15-17 → Railgun S 21-24 → Index 18 → Index II 1 → Index 19-24 → Index II 2-7 → Endymion → Index II 08-24> Index III
https://toarumajutsunoindex.fandom.com/wiki/Anime_Timeline
