Can someone perform DDoS on my VPS even if I have not installed any webserver software like apache, nginx, etc.? Since I have not installed any webserver software, it's not really available to the public. Am I right? It can only be accessed via SSH.
DDOS means Distributed Denial of Service. If no service is provided by your VPS then no service can be denied. If SSH is provided then SSH can be denied. This means a DDOS can actually work against your system too: if all the bandwidth is taken by the attack or your system is busy trying to process connection attempts to SSH then you will not be able to login anymore yourself, i.e. the service is denied for you.
Additionally your VPS shares the hardware with other VPS which all share the same bandwidth and CPU. Due to this if any of the other VPS gets attacked it might affect your VPS too.