How to apply object detection?
There are multifarious studies about object detection. The method you are going to use depends on your data. For instance, if you have centre of the objects alongside their height and width, you can simply use YOLO method which is one of the pioneer approaches for detection. As another example, If you have data which is classified pixel-wise, you can employ RCNN methods or Fully Convolutional methods like FCN itself or SegNet. There are open source implementations with appropriate trained parameters for these models on GitHub.

Apart from the methods, you have to consider the restrictions they may exist for your conditions. For instance, there may be situations where your task is online and you have limited computational power. In such cases, I don't think you can employ very deep architectures like SegNet.