1. What is the significance of the ornament I see on the doors of Jewish homes?

It is called a Mezuzzah, and is a small case containing two parchments on which are written passages from the Torah.  It is not some 'good luck' charm or just a decoration, but it is a constant reminder of G-d's presence and His commandments.

The mitzvah (commandment) to place mezuzot on the doorposts of our houses is derived from Deut. 6:4-9.  In that passage, G-d commands us to keep His words constantly in our minds and in our hearts, by (among other things) 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. The scroll is then rolled up placed in the case.  The case with its scroll is then nailed or affixed to the right side doorpost at an angle, with a small ceremony called Chanukkat Ha-Bayit (dedication of the house). A blessing is recited.

Every time you pass through a door with a mezuzah on it, you touch the mezuzah and then kiss the fingers that touched it, expressing love and respect for G-d and his mitzvot and reminding yourself of the mitzvot contained within them.

The word Mezuzzah is Hebrew for "doorposts".

2. I'm a Jewish teenager and I've worn a Kippa since I was a child but nobody every explained why.  Can you write something about it?

I'd be glad to and I'm glad you asked!  It's important to know why we do what we do, or else we run the risk of meaningless observance.   Hashem wants us to serve Him with kavanah ( dedication) which requires some understanding.  So, your question is a great one!

It was an ancient practice for Jews to cover their heads during prayer, probably derived from eastern cultures in which it was considered a sign of respect to cover the head.  (Interestingly, in the west, it's just the opposite.  Men remove their hats as a sign of respect.)  At any rate, by covering their heads, Jewish men showed respect for Hashem.  Besides that, in ancient Rome, servants were required to keep their heads covered, while free men did not.  The Jews, in covering their heads, showed that they saw themselves as servants of Hashem. Later in the Middle Ages, Jews covered their heads as a sign that G-d was always above them. 

Wearing a Kippa is a custom rather than a commandment, but a custom with signficiant meaning.  Kippa is a Hebrew word while yarmulke is Yiddish.

In our modern world, wearing a Kippa is still a testimony of one's dedication to the G-d of Israel and His Torah.  Wear it in good health!  And wear it with devotion!