The main thing you need to do in order to write a topic sentence for your essay on globalization is to have a very clear idea of your focus. Often, the first sentence of an essay is the last one you should write, because it signposts where the essay will go, something you often don't know until late in the writing process.
The key to a good focus is to narrow your topic as much as possible. One reason you may be struggling, in fact, is that your topic is too broad. You won't be able to cover all the aspects of globalization in one essay, and so you might focus, for example, on local consequences, and create a topic sentence such as "There are positive and negative effects of globalization on teenagers in [name of the town where you live]".