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. Also, reading through the Walkthrough might give you some extra overview.
The reference documentation provides an overview of the main functions in FieldTrip. Furthermore, there are specific documentation sections related to the intergration with external software.