Danno, You wouldn't be the eldest child would you?
I had the same problem in my teen years. And it was my mum who was the worst.
She freaked out when I started going out, driving, drinking etc
Basically, I just took it easy for a while to build their trust, and let them know that I wasn't an irresponsible jerk.
Then when they asked about what I was going to do, and who I was going with etc, I would just exaggerate it to them and let them freak out so then when I told her the truth, it wasnt so bad.
eg:
Mum:What are you doing? Who is going to be there? Will there be drinking? Will there be parents etc?
Me: Well, Im going to this dudes 21st, so TECHNICALLY there will be adults there, there will be a LITTLE bit of drinking, and I have heard rumours that there will be strippers, but I'm sure everything will be fine.
Mum: *Freaks out*
Me: Relax, I'm having a couple of beers with mates and watching the cricket. Chill out.
If you are going to act guilty and sneak around, they are going to treat you like you are guilty.
Best policy is being cool, honest and just telling them that you know they worry, but you will be fine.
It sounds stupid, but generally, the more you tell your parents about what you are doing, the less they really want to know.
You'll get there.