NextCloud is fully functional out of the box.
Just connect an ethernet cable, then boot up.
On the screen you will see the IP address (My IP address is xxxxx)
From your desktop PC type the address in your browser to access your cloud.
If you want to connect wirelessly that is a bit more complicated but you can look up step to step instructions in Raspbian on Google.
exactly. I recommend that your read the instructions carefully. Normally the place to start is the command
you can look up online any guide about getting started with Raspbian for the details.
1) connect ethernet
2) write down IP address
3) access from your laptop/phone/android app/tablet/desktop to your cloud using that IP
There is nothing really that you have to do, it is already working.
User is admin, password is ownyourbits
I recommend you change that and also the PI user password in raspi-config