Question: I already got a plan, a storyline and everything down in my head. I'm 12 but I don't know how to get publishers interested in my story. I'm also writing it down on Wattpad. :/Answer:
Don't get too far ahead of yourself. Your job right now is to write the story. That's enough of a task to be getting on with.
It may take you a matter of a few months, or even a few years to finish a novel, and there are a lot of challenges you may find along the way.
After you've written your first draft, then you have the task of revising. Most books need to go through several rounds of revision that cover everything from fixing plot holes, cutting parts that aren't crucial to the story, fixing inconsistencies in the plot and characters, making scenes more dramatic, and finally to things like fixing grammar, spelling, and punctuation mistakes.
Once you have a polished manuscript that is as good as you can make it, then you can start looking for publishers. Here's a description of the basic process...
Generally, you would create a short list of promising publishers and/or agents and send them short query letters that tell them a little about your story and you. If you're lucky and they're interested, they will ask to see the manuscript.