First of all, you can find some great maps of Numenor, Middle-earth and the Undying Lands here:
http://www.tolkien-archives.com/maps.shtml
Numenor was an island made by Iluvatar for the race of men to live on. It was made at the beginning of the Second Age, and men settled on it soon after. Many kings ruled there, but eventually one, Ar-Pharazôn, captured Sauron, and the Dark Lord urged him to break the Ban of the Valar and sail into the Undying Lands to the West. The King set sail with a great fleet towards the Deathless lands and landed on Aman. This was against the "laws" of the Valar. Iluvatar then created a great chasm in the sea, and the entire fleet and the island of Numenor was cast down. You can find the entire account in the chapter AKALLABÊTH, in the Silmarillion. Any more questions, just ask.