Google Docs if you are Google-centric
LibreOffice or OpenOffice are also free.
All three have a whole suite of apps, like Word processing, spreadsheet, etc.
Personally, I get the annual Office365. I can use it on 10 devices (like my iPhone, iPad, PC & Macs) and know it works perfectly for the whole family.
If you just need simple word processing, just use the builtin WordPad.