That is a tough question to answer. All depends on what router you have. I will say, you should know the answer to this question. No judgement here, you just really shouldn't run your router with a default password. That makes it really easy for hackers to find their way into your network. You don't need to be a network expert, just look for the model of your router (probably on the bottom, near the serial number) and google it with "default password" and you'll undoubtedly get a lot of pages explaining how to do what you want. There are many levels of security you can look into but the biggest "bang for the buck" by far, is changing the default password.
While you are at it, the second big thing you should look into is updating your router to the latest software. Security patches come out frequently and running the latest version is always worthwhile. Most routers make this really really easy. You click a button (something like "check for updates") and it does it all for you. Again, you don't need to be an expert to do this.