In order to write a good topic sentence, the first thing you need to know is what you are going to say on the topic. You have to do this first -- you can't write a good topic sentence if you don't know what your argument will be.
So, imagine that you are going to say that nuclear energy is a good idea for economic reasons. Your topic sentence could say "With the rising price of oil, there is a need for more economical sources of energy such as nuclear power."
Now imagine that you want to promote nuclear energy for environmental reasons. You could then say "Although many people fear nuclear energy, it is much better for our environment than fossil fuels are."
Or if you intend to talk about both reason, you could say "Nuclear energy is superior to fossil fuels because it can save our economy money while reducing emissions of greenhouse gases that cause global warming."
In all of these cases, you are telling the basic idea of what you are going to argue. That is the main thing that you must do in a topic sentence.