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>>283294889
If you're only interested on the franchise because of Railgun and Misaki, it's better if you watch the prior two Railgun seasons that established the setting and characters, but she's a main character starting with season 3 (titled Railgun T) for the anime, and if you want to know further read from chapter 41 of the Railgun manga (IIRC) after which she becomes a regular. For Astral Buddy and Mental Out, where she's a main character in both, you can read them each pretty much as standlones, but it's better if you read at least Railgun first.
https://mangadex.org/title/e10c02da-e7a3-40e2-8b3d-c6f769d4618c/toaru-kagaku-no-railgun-gaiden-astral-buddy?tab=chapters
https://mangadex.org/title/4e0ab019-afcb-4fd9-98eb-fcb28cf7fc81/toaru-majutsu-no-index-gaiden-toaru-kagaku-no-mental-out
Lastly, there's at least one novel from the main series that you should read where Misaki's the main character and I heavily recommend as well, and it's To Aru Majutsu no Index: New Testament volume 11, which has a lot of background about her relationship with the novel's MC. Enjoy anon.