Hello hello!
Now, I have been pondering over this for ages. I've been wanting to publish a story. However, there are two problems with this: Firstly, I am quite young, I am still in secondary school (British) and so I'm scared that the publishers won't take me seriously. And secondly, I have written many, many stories over the course of my life, but I have only ever legitimately finished one on account of my perfectionism. I would really appreciate some advice on how to get stories done and also how to work up the courage to send something in to be published. Thanks.

Do you have a Language teacher that could support you? I think that's a good first step; Having an adult sponsor you to help you get started. Having an older person with credentials back you up might help persuade publishers.

You should join some amateur writing contest. People sometimes get published by winning a contest. But first, you should ask someone to proofread your work. That person should be someone whose major is literature. That way, you can gurantee to win even just consolation prize. You could also post your work online. There are lots of site for that. They can give you reviews, and helps you find some grammar mistakes if you have one. That way, you can be confident of your work, before you publish it.