Maybe you've heard of a human rights abuse case that really fired you up. Writing a letter to a politician may seem old-fashioned or pointless, but it's actually a good way to get your voice out there, and get your case heard. Politicians, especially local ones, rely a great deal on letters from the public when it comes to decision making. They obviously want to make you happy so they will be reelected (although we're not trying to dismiss the possibility they may actually care, too!) or for other reasons. In any case, they will be made aware through your letter of a case they might not have known about before, and that is the first step in creating change.
Here's a website you can visit to look up any US governer, senator and representative.
Here are some tips to help you promote your cause. If you scroll down, there is a separate section on contacting representatives. There are many more tips too, like information on meeting your Congressperson.