In Word, Excel and PowerPoint, a new document (or workbook, or presentation) opens automatically when you start the program. You can just start typing or inserting content into it.
You can also create additional new documents. An easy shortcut to do so is to press Ctrl+N. You can also choose File-->New. That latter method has the advantage of opening a selection of templates you can use to jump-start your work if you don’t want a totally blank document to start with.