First login as the "oracle" user, and issue the command "sqlplus "/ as sysdba"" (I am assuming that you already have configured the 

PATH environment variable correctly) and then type the following:

alter tablespace tablespace_name 

add datafile '/oracle/data/file1.dbf' size 12G autoextend off;

The above command adds a data file named "file1.db" of size 12 GB to the "/oracle/data" path with the autoextend feature turned off

Source: http://www.adp-gmbh.ch/ora/sql/alter_tablespace.html