Right so this morning I tried to add a security pin to my LG Viewty (on Orange network). I added what I wanted my pin to be but then it asked for my PUK code? which I don't know. I turned my phone off hoping that it would just delete what I'd done and then I turned it back on and it said Invalid USIM card!
Help me please. I don't know how to get my phone working again

You'll need to phone Orange to get the PUK code.

Thank You.
As I'm under 18 am I allowed to speak to the customer services because do they ask for details or will they just want my mobile number etc?

You should be okay. If there is a problem get a parent/gardian or another responsible adult to speak to them.

Cheers
Rang them earlier on and they were quite helpful, and ran a check and found the sim card is faulty so are sending me a new one which should be here tomorrow.