1. Don't panic
Let us start with the infinite wisdom of Hitchhiker’s Guide to the Galaxy: DON'T PANIC. YOU ARE FINE AND EVERYTHING IS OK.
Most symptoms of "greening out" (imbibing too much cannabis) will dissipate within minutes to hours, with no lasting effects beyond a little grogginess. Give it some time and these feelings will eventually pass, trust us.
Also, contrary to what you may have heard, there have been zero reported cannabis overdose deaths in the history of ever, so despite how freaked out you may feel or how sweaty you get, you won't expire from excessive consumption. (Don't take that on as a challenge, just keep in mind that if you accidentally overdo it, you'll be OK in a while.)
2. Know your limits before consuming
If you can, try to prepare for your cannabis session according to your tolerance level. Consume with friends you know and are comfortable with, and don't feel pressured to consume more than you can handle. It's all well and good to make new friends, but being surrounded by strangers when you can’t feel your face is unpleasant at best and anxiety-ridden at worst.