fess up, thats all you can do, if you want to lessen the damage go with the " I was curious, but then I got to scared to even try one" but can't garuntee that will work. if it wasn't your first time do not admit to anything that they havent accused you of. only admit to what they allready know. I hate giving this advice really, I have kids that will be teenagers in a couple years.
I know when I was living home if my parents would have found anydrugs they would not even ask where I got them...unless they wanted some. They are quite familiar with what happens to snitches, especially my dad being in the "hoticulture buisness" when I was young