How to configure POSTGIS?

What do you mean by configure?

Postgis installation provides you a set of functions and types you can use to store spatial data. There is no configuration necessary. All you need to do is use the functions and types to process and store your data.

So once you have a spatially enabled database (aka database that contains all the objects added by postgis), which you can create by using postgis template database, you can create tables as shown below.

As you can see I have a geometry column (geom_col) and several check constraints that ensure correct geometry is stored. These check constraints use some of the postgis functions (ndims, geometrytype, srid).

CREATE TABLE spatial_test
(
  id integer,
  geom_col geometry,
  CONSTRAINT enforce_dims_geom_col CHECK (ndims(geom_col)=2),
  CONSTRAINT enforce_geotype_geom_col CHECK (geometrytype(geom_col)='POINT'::text OR geom_col IS NULL),
  CONSTRAINT enforce_srid_geom_col CHECK (srid(geom_col)=26918)
);

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I run this table script on PostGre window it gives me following error.

ERROR:  type "geometry" does not exist
LINE 4:   geom_col geometry,

Thanks
dAshish

Ok, so that means that the database you are trying to create the table in is not spatially enabled.

Make sure the database is either created from the postgis template, or alternatively you could run the following scripts to generate all necessary functions and types:
<PGHOME>\share\contrib\lwpostgis.sql
<PGHOME>\share\contrib\spatial_ref_sys.sql

If the database is spatially enabled, you should see spatial_ref_sys and geometry_columns tables in your public schema. You should also have a few hundred functions having to do with geometries.

I have installed PostGre on linux machine, and working on the same machine

Now could you please advice me how to make the database spatiall enabled.

I tried to run this
<PGHOME>\share\contrib\lwpostgis.sql
<PGHOME>\share\contrib\spatial_ref_sys.sql
It gives me error "Could not save history to file "hare": Permission denied"

Thanks,
dAshish