Type:
dbaccess your_database_name (of course replace your_database_name with name of your database)
Press "Q" (without quotes, of course - Q stands for "query-language")
Press "N" (without quotes, of course - N stands for "new")
Type in your query. For example:
select idxname from sysindexes;
After you are finished with query press ESC (escape key)
Press "O" (without quotes, of course - O stands for "output")
Press "N" (without quotes, of course - N stands for "new-file" - you have to be carefule because if file exists it'll be overwritten)
Type in file name, for example:
my_first_output_file
Press Enter key when finished
Press "R" (without quotes, of course - R stands for "run")
Press twice "E" (without quotes, of course - E stands for "exit")
And you're back in OS an you have your file.
HTH