Internet of Things (IoT) is an emerging concept which aims to connect billions of devices with each other. The IoT devices sense, collect and transmit important information from their surroundings. This exchange of a very large amount of information amongst billions of devices creates a massive energy need. Green IoT envisions the concept of reducing the energy consumption of IoT devices and making the environment safe. Inspired by achieving a sustainable environment for IoT, first give the overview of green IoT and the challenges that are faced due to excessive usage of energy hungry IoT devices. The researcher then discuss and evaluate the strategies that can be used to minimize the energy consumption in IoT like designing energy efficient data centres, energy efficient transmission of data from sensors and design of energy efficient policies etc. Moreover, critically analyse the green IoT strategies and propose 5 principles that can be adopted to achieve green IoT. Finally,  consider a case study of very important aspect of IoT, i.e., smartphones and  provide an easy and concise view for improving the current practices to make the IoT greener for the world in 2020 and beyond.