If you are using Windows, use "Start", "Find", "Files and Folders" to find a file in your windows directory (or WINNT\system32\drivers\etc) called "hosts". Verify that the file is not "read only" by right clicking it, and choosing it's Properties. Then open the file for editting with Notepad. There should already be an entry for "localhost". Follow that format when you insert your domain and our IP.
On Windows98 and Windows95, the order may be 'hostname' then 'IP address'. On Windows2000 and WindowsME, the order is 'IP address' then 'hostname'
Then save this altered hosts file and close notepad. Make sure Windows did not silently save the file as "hosts.sam". The filename has to be "hosts". You may also need to reboot for the change to take effect. Next time you try to go to "www.example.com", your browser will try to find that domain at the corresponding IP instead of looking up the IP through DNS.