The only rules about building a Sukkah are that it must have at least three sides, and we must be able to see through the skhakh (the "ceiling" of the Sukkah), so that the stars are visible at night. 

A  skhakh is traditionally made up of palm fronds, but can be made from the branches of any tree, as long as it is live when it is cut.

Some people have a tradition of building a Sukkah so that it resembles one of the letters of the word Sukkah.  The Samech, 
() is a closed Sukkah.  It has a moveable side (usually a sheet) for a door. 

The Kaf () is a three-sided Sukkah. 

The Hay () is a Sukkah that has a partial opening on one of the three sides.

Many people like to decorate their Sukkah. They use their Rosh Hashanah cards up, and decorate with fruit and other things. This tradition helps make the Sukkah a pretty and fun place to be.