Hi again ruspinigirl!
It's very hard to find out what is being cast for TV and film without an agent, so you can certainly send your details to agents and see whether anyone is interested. You could also look out for information on student films (which don't normally get cast via agents, as there is rarely any pay) on sites like Mandy.
There is a book called "Contacts" published by Spotlight which lists most agents in the country and has a separate section for children's/young people's agencies. Some libraries have copies - you might even find that your drama teacher has a copy - otherwise you can get one on-line direct from Spotlight or from a site like Amazon. Some larger bookshops also carry it.
Another useful book is "Actors' Yearbook" which gives contact details for a number of agents and also gives details about how they prefer to be contacted. Another you might find interesting to read is "So You Want to Tread the Boards?" by Jennifer Reischel which gives lots of useful advice for those considering entering the profession.
Just because an agent is listed in Spotlight or Actors' Yearbook, it does not mean they are good by the way - if you are offered representation, make sure you talk it over with a knowledgeable adult. Don't join an agency if they will take you on without meeting you - they are unlikely to be worth joining.
You can send your CV, photo and a covering letter to a selection of agents and see what response you get. Don't pay any kind of up front fee to an agent. That's a really useful rule of thumb! An agent should earn money by taking a commission from the work you do.
I'd still go back to the practicalities in your case, though I know I'm going to sound like a very boring wet blanket!
If an agent did take you on (and it is "if" - because there's quite a bit of competition, especially for teenage girls) and you were (for example) called for a casting for a commercial in London - who would take you? Are they likely to be available at short notice? Would you be able to pay the travel expenses for yourself and for whoever acts as your chaperone? Would your parents or guardians be happy to take you? Can the person who takes you take time off work easily?
This site can be very useful and there are plenty of people who post on here who have lots of experience, so hopefully you will get good, practical advice and tips to consider. However, you can't enter the profession as a minor without the support of the adults who are responsible for you so it's important to make sure they know and understand what you hope to do. Have you discussed your ambitions with a drama teacher at your school and with your parents/guardians? You'll definitely need them on side!
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