TeXnicCenter manages your documents always as projects. Even if there is only one file (i.e. a letter), you have to create a project for this document. It is clever to create a directory for each project.
A project contains the following informations:
You should put all files, belonging to a project, in the project's directory.
One of these files is the project's main file. The main file is the file, that
is passed to the LaTeX-compiler. If you have split your document into several
LaTeX-files, they should be included into the project's main file with the
\input
or \include
command.
TeXnicCenter needs to know, which file is the main file of a project. You can specify the main file, when creating the project, or you can change it later, by choosing 'Properties...' from the 'Project'-menu and changing the 'Main file'-path.