On the doorposts of traditional Jewish homes, you will find a small case. This case is commonly known as a mezuzah (doorpost),because it is placed upon the doorposts of the house. The mezuzah is not, as some suppose, a good-luck charm, nor does it have any connection with the lamb's blood placed on the doorposts in Egypt. Rather, it is a constant reminder of G-d's presence and G-d's mitzvot.
The mitzvah to place mezuzot on the doorposts of our houses is derived from Deut. 6:4-9, a passage commonly known as the Shema. In that passage, G-d commands us to keep His words constantly in our minds and in our hearts by writing them on the doorposts of our house. The words of the Shema are written on a tiny scroll of parchment, along with the words of a companion passage, Deut. 11:13-21. On the back of the scroll, a name of G-d is written. The scroll is then rolled up and placed in the case, so that the first letter of the Name (the letter Shin) is visible (or, more commonly, the letter Shin is written on the outside of the case).
The case and scroll are then nailed or affixed at an angle to the right side doorpost as you enter the building or room.
How to hang a Mezuzah
also a great video is https://www.youtube.com/watch?feature=player_embedded&v=hz2uJJw5JSM
What is the proper placement of a Mezuzah?
The Mezuzah Scroll should be placed in a protective case. Be sure that the letters Shin-Daled-Yud on the outside of the scroll are facing upwards.
A. The Mezuzah Case is affixed on the right doorpost as one enters.
B. Its height should be just above the two thirds mark of the doorpost (about shoulder high).
C. Slant the mezuzah slightly with its' top pointed towards the inside of the room.
D. The Mezuzah should be within the outer 3.2 inches of the doorpost width.
For Mezuzahs of every kind and a great price
Here is our Mezuzah