F*** 'em. They probably won't do anything and even if they wanted to they probably don't have the resources.
But for the sake of the question, you should start at the router.
Close all unused ports, use shields up!
https://www.grc.com/shieldsup to check and see what other people can see from your connection.
Some routers have a DDOS prevention/mitigation option, turn that on obviously.
Turn off "respond to WAN ping"
A DDoS attack attempts to push you off the internet by literally flooding you with data. A more powerful system can cope with the load better.
Now, I doubt this will happen, but if you do come under a serious DDoS attack, start logging the requests coming in. If it goes on for long enough you can get your ISP involved...