SELECT * FROM DBA_DATA_FILES WHERE TABLESPACE_NAME = 'my_tbs' ORDER BY FILE_NAME;

ALTER TABLESPACE my_tbs ADD DATAFILE '/path/to/my_file.dbf' 
    SIZE 100M AUTOEXTEND OFF;
-- Autoextend with max size:
ALTER TABLESPACE my_tbs ADD DATAFILE '/path/to/my_file.dbf' 
    SIZE 10G AUTOEXTEND ON NEXT 1G MAXSIZE 32000M;    
-- For TEMP tablespace:
ALTER TABLESPACE my_tbs ADD TEMPFILE '/path/to/my_file.dbf' 
    SIZE 100M AUTOEXTEND OFF;