I’m just posting my answer here, if any of you will find it helpful? :) Let me know, here are my main tips. :)
“Hey!
First of all, thank you, it's so nice that you messaged me :)
And I would be glad to help you :)
1. The plot. So, you've told me you already have one, and that's great. Just don't get over your head with details and ending of your stories, I think they just have to naturally come out during your story, you don't have to have everything planned from the start to the end! So, don't over think it!
2. The name of the story? :)
3. Find places you'd be comfortable posting your story in! :) For example, I choose Fanfiction.net., Ao3 and Tumblr. :) (It's always a great way to meet amazing people, who are reading your work :) )
4. Characters' backstories... Yeah, I know I said don't over think it, but actually really knowing your characters well is very important... :) So, the easiest way is just...make a list of your characters and write down their main features.
5. Well, who is telling the story? Are you an all knowing writer, or maybe it's from someone POV, or maybe you'd like to do it in a way that every new chapter is from a new person's perspective :) So, think about that.
6. Decide if you want the readers to know backstories and etc immediately introduced to them, or if you want them to know what's happening during the story/chapter? (This is more particular to your question, since you asked for a piece of advice on how to START your story.) I, personally am suggesting that you just start easy, with a conversation, or an action, anything. In my opinion it's always better to let readers find out about the plot more when they are reading it from chapter to chapter, and not all given to them from the start. :) Leave a bit of mystery. :)
7. Ease into it... ;) Don't start your story with immediate action. Start with something simple, EXAMPLE, getting ready for THAT ACTION, or thinking about it. Dive in slowly :) That being said, people won't get interested in a story if the first thing they read will be nothing intriguing. So it's really just about first impression. Ease into it, but don't do it in a boring way... :) Write something that you'd think catch an eye of a reader.
Basically, that's how you start a story. You will get reviews and opinions after the first OR the second chapter (that's most likely) and then you'll just know what to write. :) It gets easier from there, you just have start. Simply, JUST START WRITING! Believe me, your writing will progress and your readers will inspire you!!!
Anyway, thanks again, I know I made this long :o But I really really hope this helps you :) WRITE ME IF YOU HAVE ANY OTHER QUESTIONS! :))”