Firstly, determine the source of the attack if possible. If the attack appears to be originating from a single source, try to block that IP address

1. Identify the vulnerable aspects of your network and secure them.
2. Reduce the load on your internet connection and limit incoming connections. This might include implementing data rate limits, blocking unused ports or services, or disabling unnecessary applications or protocols.
3. Check your firewalls are properly configured.
4. Look into using a DDoS protection service.

Be sure to save all logs and information related to the attack, as it may be useful for reporting the attack to the authorities and/or tracing the source.