Try free software such as TextEdit and OpenOffice; if those don't work for you, you need to buy Office 2008 or newer. This is usually because 10.7 and 10.8 don't support PowerPC applications such as Office 2004.
Sun/Oracle has abandoned OpenOffice. LibreOffice is now the preferred app and offers the best Word compatibility.
If you want to run Word again on the newest cat, you need to have at least Office:Mac 2008, preferably 2011.