Before you start to write you need to consider what narrative perspective you're going to take - whose point of view you will present in your story. This is usually done in the first or third person.
If you want to tell the story from the point of view of one of the characters, you can write in the first person, using 'I'. This puts the reader inside the mind of that character, seeing things as that character sees them.
The character you choose to tell the first person narrative is important. A story about bullying, for example, will be different depending on whether you tell it from the point of view of the bully or the victim.
This is where you tell the story from the point of view of someone not in the story, using 'he' and 'she' throughout. This means the story can be told from a more objective point of view.
You need to decide which point of view to take, depending on which is best for your particular story. Sometimes if a story is not working it can help to change it from third to first person narrative or vice versa.