how much acting experience do i need before i can get an acting agent in Scotland?

[Cogito]

It's the same all over the UK - you'd need to graduate from a major drama school or university Drama course.
School stuff doesn't really count for much, I'm afraid.
And talent shows may be fun, but they don't count as stage experience, nor do choir shows.

Aim for getting a place at Drama School - you can find details on http://www.drama.ac.uk.

But until you've graduated, no genuine agent would even consider you.
Forget Facebook, bebo, etc. You'd be wasting your time.
And all those casting websites are also useless. All you'll find is 95% scams and 5% amateur stuff.
The only way to find genuine auditions is through an agent.

Get experience in anything not connected with a school, and join a good acting class - again, not a school one.
But honestly - drama school is essential if you want to become a professional.
Good luck!