I can't get the motors to run and/or can't arm the ESCs
This can be due to a number of mistakes:
Is your APM slider switch in Flight Mode (away from RC pins)?
Are you in Acro, Stabilize or Simple mode? As a safety measure, the motors won't arm in other modes.
Are you sure you're arming them right? When the motors are armed, the green LED on the IMU will turn solid after the IMU calibration flashing (about 5 seconds). Review the arming guide to make sure you're doing it right.
Is your Yaw channel reversed on your transmitter? Go through the radio test process in the Mission Planner and check. Also, try moving the Yaw stick to the far left, instead of the right. If that works (green LED tuns solid after the IMU calibration, your Yaw is reversed.
It's possible you've changed your throttle trim settings since you did your initial radio calibration. Trim the throttle down a few notches and try arming again.
Are you flying a Mode 1 transmitter? If so, the throttle is not on the Yaw stick. Make sure it's fully down when you're pushing the Yaw stick to the side.
Some ESCs have a minimum low threshold before they will arm. If you need to, trim down your radio throttle and redo radio calibration.
Is your RC throttle channel reversed? Connect one of the ESCs directly to your RC receiver channel 3 and plug it into a LiPo (keep the prop at a safe distance!) and try it manually to be sure.
Redo your radio calibration in the Mission Planner or CLI. Make sure that the throttle channel (Ch 3) is being read properly and the values are around 1100 (low) and 1900 (high), more or less. Remember that your LiPo must be plugged in to power the RC gear.
If you've checked the previous things, it may be that you need to recalibrate your ESCs. Do it the manual way (second method here to be sure).