If it's your first time, an eighth should do you just fine. I recommend not eating for about 6 hours before hand, and since it's your first time, not smoking weed until you're on your way down. That way you'll be able to experience an undiluted trip and get to know mushrooms. I would recommend smoking when you plan on going to sleep, as mushrooms are notorious for keeping you awake long after they have worn off. Otherwise, have fun, mushrooms are by far my favorite psychadelic.

You must be somewhere warm if'n your planning on tripping in the woods, it's fucking cold here!