Again, it's a Windows and not a Word thing. Control Panel > Regional
and Language Settings > Languages > Details > Default input language.
Or, if you only want Word (and not all the other programs) to use
Japanese input, then when Word opens, go to the Language icon at the
bottom right -- if Japanese is installed, it will be there -- and
change it from EN to JP.
Or, if you're saying that your computer starts with Katakana and not
Hiragana, I don't know; none of the instructions give that as a
possibility. Maybe you have a Japanese program not provided by
Microsoft?