When you plan to do your work on a future date, try to take all of the elements of the assignment into consideration. How long do you think the research will be? How long do you think it will take to read everything? How much time should you spend planning, writing and revising everything?
By taking all of these elements into consideration, you will have a rough idea of how much time it should take you to complete the work. This may not come naturally at first, but the more you do it, the easier it will become. If you are really having a hard time, ask a parent or teacher to help you out with the first few assignments until you start getting the hang of it.
When you know approximately how long it will take to complete the assignment, then you can break it down and work on it in chunks, making it much easier to actually get started and get it done.