Sort of related to what traveller said, I wanted to say a little bit about “viewpoint” characters and deciding what is “on camera” vs. “off camera”.
There are three “voice” styles. First person is, “I went to the store.” This makes it very clear who is speaking, and who your viewpoint character is. Second person is, “You went to the store,” or “You are on your way to the store.” (Second person is not used much except in Choose Your Own Adventure books :) And third person is “Joe went to the store.”
Most of us will probably write in the third person. With third person, you have to give the reader some cues as to who the viewpoint character is. The viewpoint character is usually your main character, but you can sometimes switch to another viewpoint character if you want to show something else happening out of sight of the main character. The best time to do this is at a chapter break when you would naturally change scene anyway.