There's 2 ways to play Iron Bull: Defensive Reaver (picking up defensive skills in Vanguard and mixing them with Reaver skills for better survivability) or to go all out DPS by going full Reaver + Two-Handed and Battlemaster.
For the first option, you would get almost everything in Vanguard so you would have 3-4 skills that generate Guard to go along with your Reaver DPS abilities. Personally I'd pick up War Cry, Charging Bull, Unbowed, and all of the passives that give you more armor/survivability, and use that along with your reaver skills.
For the second option, here's the build I'm currently using (Full DPS Reaver).
Whatever you end up doing, make sure to craft stuff with guard on hit, it REALLY helps Iron Bull survive considering how low his health will be from using reaver abilities. Also, the + Max Stamina amulet is really useful so you can use Ring of Pain and Devour more often.