Every time Microsoft bring out a new edition of Word, they seem to change the way that things are done. However this link covers all of the more recent versions of Word:
https://support.microsoft.com/en-us/topic/create-a-template-86a1d089-5ae2-4d53-9042-1191bce57deb
So you should start by opening a blank document as you usually do. Then change the font, the font size and (if required) the margin settings to how you'd like them to appear when you open a blank document in future. (I've always found it best to actually type a few characters with the required font settings and then to delete them. That seems to get Word to 'lock onto' the settings).
Then use the process in my link above, saving the template with a name such as 'BlankDocument'. The next time you use word, go to File > New > Custom and click on that name to open a blank document with the required settings.
It can also be useful to go through the process again and create further templates, such as 'LetterHeading', 'Invoice', 'Complaint' or whatever else you might need frequent access to.
If you'd rather get rid of the template that Word uses when opening a blank document without using a template, and replace it with your own, you need to follow the above procedure, locate 'normal.dot' on your computer, delete it, put your own template (i.e. 'BlankDocument', or whatever) in its place and then rename that file to 'normal.dot'. It's a bit fiddly but it works ;-)