With this addon (and its other versions for older expansions) installed, you won't have to painstakingly memorize every boss fight. The other major addon that you shouldn't be without, Deadly Boss Mods makes World of Warcraft's complex boss fights a little more approachable by providing real-time alerts to keep you one step ahead. More advanced users will appreciate the ability to program custom macros that can change the state of your action bars too.
Bartender lets you set keybindings and position them perfectly for an optimal setup.
Before long, your hotbars are going to fill up and become an annoying mess. If you're starting out in World of Warcraft as a brand new player, this should be one of the first mods you consider. Enter Bartender, an addon that gives you full control of ten action bars including their position, size, and even transparency. By default, they're quite small, can lack crucial information, and you don't have a whole lot of customization options.
World of Warcraft's hotbars are easily the most outdated part of its user interface.