You've probably seen that sites like Wikipedia and Reddit are down today to show that they oppose SOPA (the Stop Online Piracy Act) and PIPA (the Protect IP Act). For more information on why these acts should not be allowed to pass, and what would happen if they do, please check out Google's explanation here. On Google's site you can also sign their petition and share the information with others using Facebook, Google+ and Twitter.
According to Wikipedia:
SOPA and PIPA would put the burden on website owners to police user-contributed material and call for the unnecessary blocking of entire sites. Small sites won't have sufficient resources to defend themselves. Big media companies may seek to cut off funding sources for their foreign competitors, even if copyright isn't being infringed. Foreign sites will be blacklisted, which means they won't show up in major search engines. SOPA and PIPA would build a framework for future restrictions and suppression.
You can also use Wikipedia's blackout page to find your representatives by zip code. Contact them and let them know your concerns.