Prior to this, I had been using my pre-level 10 "rotation" of Concussive Shot, Serpent Sting, Autoshot and Raptor Strike when the mob came into melee range. Back then, of course, pets on Defensive only attacked when a mob hit you... so it wasn't until the mob was half dead that my pet came to help.
From then on, it was clear to me that learning how to make sure my pet was active and attacking a mob - preferably keeping said mob off my hunter at the same time - was a priority.
So: on to the content!
How to make your pet Attack, Follow and Stay:
The 3 leftmost buttons on the pet bar are Attack, Follow and Stay in that order, with the default keys being Ctrl-1, Ctrl-2 and Ctrl-3.
Now, unless you have an unusually-shaped hand (or perhaps have long fingers), these combinations are extremely awkward to hit as keybinds.
There are 2 ways you can change this to work.
Firstly, you can alter the default options for the Blizzard interface. This can be found by pressing Escape for the Game Menu -> Key Bindings, and scrolling down to Secondary Action Buttons 1-9. Pet Attack is actually given a second default keybinding, found further down and defaulting to Shift-T, which you can change as well.
Secondly, you can use macros.
That's it for this section, stay tuned for Parts 2 and 3!