Someone else posted something like this,but it didn't cover everything.This will not cover the basics, such as shooting, for that, see the other instructable.
Step 1: Aquire the stuff
To airsoft,you need a gun,I recomend Desert Eagle for beginners then you can move into the more fancy guns or vice-verca.You dont need any armor(except eyewear)unless you're playing aginst people with high fps(feet per second)guns,OUCH!
You also need a fort for two reasons:
(1)If its camo, you can give your enemys a BIG suprise attack
(2)a retreat area. I have a trench in my backyard and its great!I recomend one.