Cortana is a built-in search engine. It is located in the bottom left corner of your desktop, next to the Start Button.
We need some more information in order to help you. What does it tell you when it rejects your password? Are you using a laptop or desktop? Are you upgrading from Windows 7 or Windows 8? Could you post your machine's specs?
If all else fails, a computer shop with a DVD of Windows 10 can probably get you into your computer.