Answer:
Manchester is one of the UK's major centres for performing arts so there are auditions going on most of the time. Any reference to specific auditions here will be outdated very quickly but here are a few ideas to help find them.
First, scan the Manchester Evening News on a regular basis for open auditions. The adverts tend to be small so a careful search is needed. Other local news publlications can turn up opportunities as well.
Second, get involved with performing arts groups. There are many commercial acting, dance and singing school in and around Manchester but some can be very expensive. Try getting in touch with the local community centres and the council to find out what publicly funded arts and drama groups are in your area. Some of them will have auditions for their own productions on a regular basis. Others will frequently get information about auditions which they will pass on to students and group members.
Finally, keep talking to people who are involved in drama and performing arts. It is important to listen for any information that people may have. The closest contacts you are likely to have as a teenager will be the staff in the school's music, drama and media departments. It's always worth talking to the teachers but don't forget the technicians that work in the departments. They often have interests outside the school and will be happy to pass on information to students who show an interest.
As well as needing a little luck, remember that it's about hard work as well.