Basic SQL commands
From Peter Pap's Technowiki
Create tablespace
create tablespace tablespacename datafile '/path/to/file/filename.dbf' size 10m autoextend on next 5m maxsize 1G;
This will create a 20mb datafile that will extend by 5mb until it gets to 1gb max.
Add datafile to tablespace
alter tablespace tablespacename add datafile '/path/to/datafile/datafile