How do I Create Tablespace in Oracle 11g?
The answer to your question can be found in the SQL Language Reference (excerpt follows).  


  Creating Basic Tablespaces: Examples 
  
  This statement creates a tablespace
  named tbs_01 with one data file:
  
  CREATE TABLESPACE tbs_01 DATAFILE 'tbs_f2.dbf' SIZE 40M     ONLINE;


It sound like you are new to Oracle databases.  Oracle provides a wealth of documentation at http://docs.oracle.com/en/database.  Specifically, I recommend reading their excellent introduction to the database called the Concepts Guide.