You find the class AbstractRobot here, which provides the interface fulfilled by all the robots (which are derived from this class). There are two robots which are recommended as a first example: lpzrobots::Nimm4 and lpzrobots::Sphererobot3Masses.
Also the simulation template_sphererobot with ode_robots/simulations/template_sphererobot/main.cpp as a main function are really good commented and therfore recommended as first example. You can easily find them by clicking on the tab "Examples" at the top of the page.