This should be a direct topic, as there is much information on the topic. As with all essays, I think that the first step is to check with your task description and instructor to make sure that you have included all the necessary parts of the paper. The next step would be to take a position on the topic in terms of what you believe and what you think the evidence indicates about global warming. Establishing this will be important. For example, are you going to argue that global warming is a concern or something that has been manipulated to be seen as one? Are you going to argue that it is caused by human made factors or that it is a natural climate process? I think that these positions are, infact, positions on the topic and can be defended with evidence. Arranging and understanding the evidence out there and what you are going to say about it will be of vital importance in the paper.