The documentation for FieldTrip is split into several parts:
The tutorials will help you to get started by providing examples that you can copy-and-paste into Matlab. The introduction tutorial is the best place to start if you do not have any idea about the ideas behind the FieldTrip toolbox. The Walkthrough will give you a good overview of the conceptual ideas behind the toolbox and analysis methods.
The frequently asked questions section provides a lot of practical information. Furthermore, the example MATLAB scripts contains pieces of documentation that are often not so elaborate as the tutorials, but that go in more detail into specific aspects of the data, code or analysis.
If you are very eager to get started with your specific data, please proceed to the user documentation section on importing your own data.