select * from users
save D:\test.sql create;
Use the spool:
spool myoutputfile.txt select * from users; spool off;
Note that this will create myoutputfile.txt in the directory from which you ran SQL*Plus.
If you need to run this from a SQL file (e.g., "tmp.sql") when SQLPlus starts up and output to a file named "output.txt":
select * from users;
sqlplus -s username/password@sid @tmp.sql > output.txt
Mind you, I don't have an Oracle instance in front of me right now, so you might need to do some of your own work to debug what I've written from memory.