In Javascript How can I customize alert window?

In a static html page i want to do following actions:

1. as the url is entered first I want to display the terms and conditions using window.alert("TERMS AND CONDITIONS").
2. In the dialogue box I want two buttons "agree" and "dont agree". If agree is pressed I want to use window.location.assign("next page url") .And if dis agree is pressed I want to use window.close("","_self")

I'm a total beginner and have minimum knowledge of Javascript.

Thank you in advance.