Read all of this, step by step: Mandatory Hardware Configuration — Copter documentation
Then go through this: First Flight with Copter — Copter documentation
Even if you think a section doesn’t apply to you–read it anyways. There’s a trove of information in the wiki. If you struggle to understand something, keep on reading and come back to it later.
When you feel like you understand more than 50% of it, start at step one again, and this time configure your drone.
And when you feel somewhat comfortable with the workings of Ardupilot, you can fully dive in and properly tune your drone with this: How to methodically tune (almost) any multicopter using ArduCopter 4.4.x
Ardupilot has a very steep learning curve, but just push ahead and it will start making sense at some point. Also just randomly read through the forum! Lots of unsorted information contained here as well.