We are told in the Pasuk that we are commanded to build a succah every year because we lived in Succot in the desert. Their are two different views about what this means. Some have said that when they came out of Egypt the Jews in the desert lived in Succot. Other explain that G-d made a Succah with protective clouds on top of the Jews. The roof of the Succah must be made of plant materials but these must be detached.
Many people have their trees trimmed special for the holiday.
This has a double advantage the area is made kosher for a Succah and the branches and leaves can be used for the roof. The majority of the roof of the Succah must be covered with roofing material kosher for a Succah. Many people have heard that you need to see the stars through the roof of the Succah. The way to tell whether the majority is covered is to look at the shadow cast through the roof, if you see more sun than shade then the roof is not kosher and you must add more branches. The walls of the Succah should not move with a breeze.