Get to know your audience by reading bestselling books marketed to this age range. This will give you ideas about the type of story and writing style that 7th-graders are interested in reading.
Research heavily. Children are as discerning as anybody, so it is vital that your story is as authentic feeling as it is compelling. Talk to your target audience. If you have children, nieces and nephews, talk to them and ask them to talk to their friends. Find out the sort of topics and characters that 7th-graders like to read about.
Think about your story arc. It should have a beginning, middle and end. Break your story down into these three acts. Flesh out each one with details.
Get to know your characters. A good method of achieving this is to make a list of everything there is to know about them. Think about their motivation, their likes and dislikes, where they come from, what they do for a living. Even if this information never makes it into your story, it is useful for ensuring that you know these characters intimately.
Give your characters some flaws. None of us is perfect, so it is difficult to identify with characters who are flawless. Characters in books can become role models to children, and it is important that they are realistic.
Create a protagonist and give him an interesting or exciting sounding name.
Create a problem for your protagonist that necessitates the adventure that will unfold for him over the coming pages. Make it emotive, exciting and, to some degree, believable.
Choose the right perspective from which your story is told. You have the choice of first person ("I"), second person ("you") or third person ("he/she").
Write your story. All the planning and preparation in the world will not get your story told. The only way to achieve that is to actually sit down and start writing it. Don't be too surprised when your characters and plot take you down some unexpected avenues. From time to time, good story will take even its author by surprise.
Rewrite the story through one or more subsequent drafts until you have a manuscript that you are confident is as free from errors and inconsistencies as you can make it.
Ask some friends or peers to proofread your story and offer feedback on the story and characters as well as looking out for any errors that you have missed. Write a final draft based on the feedback you received.