How to make a failover web server?

Initial data: is There a site that has two servers. One of them is nginx, MySQL, and website content (audio, photos).


Problem: When you disable one of the servers of the site should be still available.


Question: How to do it and what do I need? How can I make it so the client asked for a website and was redirected to the server B in case server becomes unavailable and Vice versa?


Thanks in advance for your answers.


PS: the OS on both servers is Debian 5.