The main thing that you need to do in order to write a paragraph about contemporary teenagers is decide what you want to say about them. You might want to look at your writing prompt to see if it gives you any information as to what your paragraph should be like. For example, should it be persuasive or just expository? I imagine that the most likely answer is that you are supposed to be making an argument in this paragraph. If that is the case, try to think about what you want to argue. Maybe you would like to argue that teens today are less motivated than teens in the past. Maybe you would like to argue that teens today have more pressure on them than teens in the past. As another possibility, you might argue that teens today are pushed to grow up more quickly than past teens. Start by picking a topic that is interesting to you.
Once you have identified your topic, write a topic sentence. In that sentence, you will state what your topic is and what argument you intend to make in the paragraph. That can be the first sentence of your paragraph. Most of the rest of your paragraph should consist of facts and opinions that are meant to back up your topic statement. In other words, if you have decided to argue that teens today face more pressure than teens in the past, you would want to support that claim. You should probably try to come up with at least three points in support of your claim. Once you have laid out your facts and opinions, you need to have a concluding sentence. That sentence will echo your topic sentence, but in slightly different words. Once you have done that, you have completed your paragraph.