Basic SQL commands

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/file/filename.dbf'
  size 10m autoextend on
  next 5m maxsize 1G;