Database cannot accept username and password

Hello,
I have installed the Oracle 10g software on my machine.
The installation process has been completed.
But I still having a problem using database. When I start SQL platform it requires username and password.
When I put the username and password, it gives invalid username and password.
However during installation I am giving the same username and password which I will enter to open the database but still I am facing error.
Please help me if someone knows about it.
Thanks
Jayme.
