Most often you won't need the arm rests. Put them all the way down.
Adjust your back support, both upper and lower parts.
Don't recline. Use the upright position.
Sit the "full" chair, with only 2-4 finger length between the front of chair and your lower leg.
Adjust the desk height so that the bottom of desk is 1-1.5 inches above your leg.
Adjust the height so that your eye is at the center of the monitor
When you lay back on the chair, you should be able to read without leaning forward
Standing itself has no evidence of being healthier than sitting. It's the stand up action that's beneficial.
Don't stand for too long (>20 min), since then your feet become tired and your posture become bad.
Don't sit for too long (>40 min). Take a bathroom break. Make sure your chair is comfortable.