1, Username and password
The default username is pi with password raspberry.
2, Enable ssh on a headless Raspberry Pi
One can put a file named as ssh and put it into the SD card. Raspberry Pi will check the file on booting. The content of the file doesn’t matter. It can be empty or anything.
3, Connect Raspberry Pi via GUI window
If you want to connect Raspberry Pi via a GUI interface. Firstly, you should install
sudo apt install xrdp
Then you can connect it with a RDP client either on Windows or Linux.