While both Carol and Robert are correct, they are assuming that you are
starting from scratch with a new document. If you have a document already
written and are now wanting to fully justify it you must first make sure that
all of it is selected.
To do this just press the CONTROL key and the A key together. This selects
the whole document. Now you can use the sugestions of both Carol and Robert
and you will end up with a fully justified Word Document.