Lots of possible methods to do that - some may be better for you.
Simplest would be to drag your document to the Dock.
Click on the document icon when you need to edit (or whatever).
If you ALWAYS start using Word with one document, then you can save your document as a Word template, and use that template as your default document.
You could also replace the "normal" template with your own template, and your Word will always use your template as the default.
I would probably have a document as a template, but leave the "normal" template as it is. You can choose your own custom template from the Word Document Gallery, as Word opens.