how can i get the password for using ubuntu software center

The passwort asked at the prompt is
a) your user password
or in the case that you are not an Administrator user:
b) the passwod for an Administrator, which can be chosen in the dropdown field.

The passwords are usually set at installation time for the first user and when the user is created for other users. If you set Ubuntu to auto-login, there is still a password for that user stored.
If you don't know the password. Follow this steps:

To see if you are an Administrator user, enter "id" in the Terminal Applicatin and hit Enter. If you see 4(adm) and 27(sudo) in the list, you are one.
If you are not, you have to ask a system administrator. You can fint he primary account on the system with:
getent passwd 1000
in the Terminal. This user is usually an Administrator.
If you have no idea who owns the account or the password is lost, follow the steps to reset it:
http://askubuntu.com/questions/24006/how-do-i-reset-a-lost-administrative-password