This is a combination of two things. First, find and rename your normal.dotm template with Word closed.
How to Find the Normal Template, See also
Why is my "Blank Document" not blank?
Second, try turning off many of the options under "Autoformat as you type." See
Weird Lines and Borders.
File > Options > Proofing > AutoCorrect Options (button) > Autoformat as you type (tab)