I installed a new Raspian Jessie on my Raspberry. After booting into it I was not able to connect via SSH. It seems the SSH server was not enabled. I could have used my TV as monitor, but there is no point as I don't even have a keyboard at home.
Therefore I asked Google and found a a solution. It involves using a card reader on a computer, mounting the Raspbian Linux partition and editing some files.
First a new empty file shall be created in
/etc/SSHFLAG (just an empty file - simply touch it).