Irene's method is the "default" way but may not always work which is why I have been using LibreOffice (OpenOffice) for years - I just find it matches my workflow with less frustration.
Should you want/need to use LibreOffice, it is used much like MS Word. You asked how to open a file in Writer (I assume you mean after installing LibreOffice)...
Click the Windows Start
icon in the lower left of your task bar to open the Windows menu. The first level menu opens.
Click on All Programs
and the second level Windows menu opens. Scroll down the program list and click on the LibreOffice
folder to expand the list. Select (click on) LibreOffice Writer
Note: if the default LibreOffice suite launch menu is open, select the type of file you wish to work with:
Text Document LibreOffice Writer
Open... (opens a generic navigation window to select a file)
opens to a blank page.
On the menu bar, click on File
then click on Open...
Navigate to the desired file. Select the file and click the Open
Your file will open in Writer
(if it is not too badly corrupt).
Fix any problem areas such as the Word TOC, page numbering, etc.
Once corrected, you can save the file in Word DOCX format from LibreOffice.
On the menu bar click File
then Save As...
At the far right of the Save as type
box, click the down arrow - the type drop list opens.
Scroll down and select (click on) Microsoft Word 2007/2010 XML (.docx) (*.docx)
The file name and type will populate the File name text box (you can change the name if you wish).
Click the Save