In order to write a topic sentence, you really need to know what you are going to argue in your essay. Your topic sentence should be a brief preview of your essay as a whole. Therefore, you have to know what you are going to say before you can write your topic sentence.
Given the topic that you mention, I imagine that you might be arguing that it is important to develop alternative energy sources because natural resources are becoming scarce. If that is what you are going to argue, you might say something like this:
In the past, people have not seen the need to develop alternative energy sources because natural resources like oil have been plentiful. Today, these natural resources are becoming scarcer and harder to obtain. Therefore, it is important to develop good alternative energy sources before we run out of the kinds of energy sources we have been using.