Problem in pgsql starting (urgent)

Hi All

I am new to pgsql and just now started with it. It is installed on our Linux Machine.
I am referring an online Doc and doing things. Now I have following prblems at my level.

when I issue following command at my prompt It gives me following error
$psql
Connection to database 'amol' failed.
FATAL 1: Database amol does not exist in pg_database

When I issued following command
$createdb mydb
Connection to database 'template1' failed.
FATAL 1: SetUserId: user 'amol' is not in 'pg_shadow'

'amol' is my actual user on linux. Now What can be the problem.. Also let me know what are the starting command on Linux for pgsql.

Amol