Hello! To start, have a traditional Big Yellow WelcomeTM:
Welcome to The Conclave!
As far as rules, there's a fairly broad selection in use by the various INQ28 groups out there, but the most common choices tend to be either Inquisitor or various forms of Necromunda - the older editions serve as the core of the fan-made Inquisimunda project (or the more official Shadow War Armageddon game), but the new version has been gaining a lot of ground, particularly with the release of things like the Venator article.
The right choice depends on what style of play most appeals to you and your group.
The Necromunda-based rules are fairly straight skirmish games, with relatively traditional competitive objective based combat.
The Inquisitor rules are a more unusual beast that is somewhere between roleplay and skirmish game. The best description I have is to call Inquisitor "tabletop improv theatre" - it's more about being the characters than you as a player trying to win.
We as a forum specialise in the Inquisitor rules, but all the different breeds of Inquisition related gaming do share their aesthetic, background and creative processes, so we'll happily discuss the hobby with people who play other rulesets.
(Certainly, my own modelling thread has some Sororitas I'm working on for Shadow War interloping in it, and I'm the main administrator here!)