Yer and like you said, it's your first time so they'll probably waiver it.
OK I know for a fact that you CANNOT go overdrawn on MyAccount from personal experience. Last summer, I didn't realise that a trip to Regent Street left me with just £3 in my MyAccount account (talk about word repetition!) and I then went into 5 different coffee shops, and my card was declined in every shop (it was not until then that I realised I had no money left!)
As far as I am aware, they are not allowed to lend to minors by law.
Since you're under 18, they can't charge you for going overdrawn. They also can't technically chase you for the £10 you owe, but if you tried that they would close all accounts you hold with them.