create user tablespace

Here is how you create users tablespace in a terabyte database in 10g :

Creating a Bigfile Tablespace: Example

The following example creates a bigfile tablespace bigtbs_01 with a datafile bigtbs_f1.dat of 10 MB:

CREATE BIGFILE TABLESPACE bigtbs_01
  DATAFILE 'bigtbs_f1.dat'
  SIZE 20M AUTOEXTEND ON;

A bigfile tablespace contains only one datafile or tempfile, which can contain up to 232 or 4G blocks. The maximum size of the single datafile or tempfile is 128 terabytes (TB) for a tablespace with 32K blocks and 32TB for a tablespace with 8K blocks

I am sorry I did not pay attention to what db version are your running?

Sir why here datafile ext is dat ok and where can set the path of that file
which location it should be created for example
C:\oracle\product\10.2.0\oradata\

CREATE BIGFILE TABLESPACE bigtbs_01
  DATAFILE 'bigtbs_f1.dat'
  SIZE 20M AUTOEXTEND ON;
Tablespace created
ALTER DATABASE DEFAULT TABLESPACE bigtbs_01;
database altered
my question now is where is this bigtbs_01 located

It should be with rest of your other db files. Do followin query to see the path; You can also give your own path while creating.
select * from v$datafile;
and look under Name column it should show you all the paths to all the datafiles.

Oh !did you mean tablespace or datafile. Tablespaces store datafiles.
I just went back to your question.

thanks everything is perfect only thing is why we are using dat ext instead of dbf

My apology it should be .dbf extension and not .dat. -TYPO :(