Here is a skeleton plugin, containing the bare minimum of code (fill in where appropriate):

#include "eboxyplugin.h"
#include "pluginconstants.h"

int ebplugin_init(void) {
  /* Change these two strings to whatever you want for your plugin (name and
   * version)
   */
  setPluginInfo("my plugin", "1.0");

  /* Do anything else here that you need to do to initialise your plugin
   * Note: if your own initialisation stuff fails here, you should return
   * a non-zero value.
   */

  return 0;     /* returning 0 means plugin has successfully initialised */
}

int ebplugin_message(int msgcode, void *msgdata) {
  /* If your plugin manages objects on the current page, you should check
   * here for when the current page/file is changed. This will also be called
   * under other circumstances (see pluginconstants.h for values of msgcode)
   */
  return 0;     /* return 0 here for future compatibility */
}

void ebplugin_deinit(void) {
  /* Any cleanup code you need for your plugin goes here */
}