By default, the user to your raspberry pi is pi and the password is raspberry.
To change the password, SSH to your device through the terminal
Enter passwd on the command line and press Enter
You'll be prompted to enter your current password to authenticate, and then asked for a new password
Press Enter on completion and you'll be asked to re-enter it to confirm it. Note that no characters will be displayed while entering your password.
Congratulations, you have changed your password!