You can't open a Word document in InDesign, but you can import a Word document into InDesign.
The very first thing you'd need to do is set-up a New InDesign document.
You are now ready to import your Word document into InDesign:
Jeff Witherell has a good tutorial video here; he uses a .rtf file, but it works the same for .docx files – he explains things really well:
How to place text from a word-processing application in InDesign | Adobe InDesign CC tutorials
Let us know if you still have more questions.