I started this show a few days ago

I was just wondering where it's story fits into relation to Toaru majutsu no index and whether its worth watching that as well

lmk

It doesnt fit into Index anywhere. Its a continuation of the side story of Railgun and yes it is worth watching. I'm looking forward to 3rd season of Index.

http://toarumajutsunoindex.wikia.com/wiki/Unified_Story_Timeline

To Aru kagaku no Railgun happens before index/on the side with Mikoto being the main character.
In To Aru kagaku no Railgun S, it tells the sisters arc from Mikoto's point of view.

Definately worth watching, imo it's better then index.