First you need to creat a world. You can make however you want. You can decide that the government is made of plant who can travel in time. You have to decide what it will be like. What type of book do you want. A thriller, a sci-fi, or a love story. This will define what your univer will look like. You can mix the style. A love story in space or a thriller in the future. once you have decide all of that you need to creat your character. Theire personality is the most important. It will define the way they react to the different event. From experience i can tell that it is easier if the main character is the same sex as you. You dont have to, but it is easier.