How to know the password for the user 'postgres'

Hello people,
I was trying to install dspace on my computer and it
required postgresql as a prerequisite. Since I am using Ubuntu so i just
downloaded and installed it using synaptic package manager. For the
installation of dspace i was supposed to exeucte the following
command..
createuser -U postgres -d -A -P dspace
but whenever i execute it, it asks for password for the new role and then
after giving the password..it says..

createuser: could not connect to database postgres: FATAL: password
authentication failed for user "postgres"

when i posted the question on dspace mailing list..one of the guy replied me
with this solution..
createuser -h localhost -U postgres -d -A -P dspace
but he said that to execute the above command i must know the password for
the user postgres..
on running this command at the end i was prompted to the enter the password
for the user postgres but i don't know the password..I think this user was
created as default at the time postgresql was installed..then there must be
a default password ...
Please help me with this problem..