sudo apt-get install postgresql-9.1 postgresql-9.1-contrib postgresql-9.1-postgis and created databse "mydb". Then extended the mydb using

psql -h localhost -d mydb -U abc -f /usr/share/postgresql/9.1/contrib/postgis-1.5/postgis.sql

psql -h localhost -d mydb -U abc -f /usr/share/postgresql/9.1/contrib/postgis-1.5/spatial_ref_sys.sql

sudo dpkg -i Downloads/postgresql-9.1-pgrouting_2.1.0-ppa1~precise1_i386.deb

psql -h localhost -d mydb -U abc -f /usr/share/postgresql/9.1/extension/pgrouting--2.1.0.sql

SELECT * FROM pgr_dijkstra('
    SELECT id,
         source,
         target,
         cost,
         reverse_cost
        FROM ways',
    15,
    16,
    directed := true)