While seniorjudge is correct in saying that suing an entity like Google over defamatory content appearing online is outrageously expensive and a lawsuit is, therefore, often not practical or worth pursuing, there are other, less expensive options available to you.
If you know the identity of the poster who posted the defamatory material, you can send a cease and desist letter to the poster (this is most effective if issued by a court but you can have an attorney draft one for you - sample cease and desist letters are available to review online). To save on the expense of an attorney, you can go to a free legal clinic or a law school for help wih the cease and desist letter.
If you wish to have the material removed from the site, properly identify the objectionable material and its location and send a "take-down" letter to the ISP (samples, again, can be found online, and help, again, can be obtained at a free legal clinic or law school). Specific information is required to locate the material. Once the notice of take-down is given, many ISPs will remove the material or disable access to the material. If the material is not removed, however, then you may need to head to court.
Here is where it can get expensive, although costs can be kept down, once again, if you can take advantage of help available at a free legal clinic.
In court, you must file suit against the poster for defamation. If you do not know the identity of the poster, you may file your lawsuit against "John Doe" and have the court issue a subpoena to the ISP ordering the release of the identity of the poster so the lawsuit can continue. The ISP will then, often, notify the poster of the subpoena prior to releasing any identifying information, so the poster has a chance to respond to the court order and potentially protect his true identity from being revealed by demonstrating a reasonable defense to the defamation claim (ie. truth, fair comment and criticism, consent, privilege).
To get your subpoena, a judge will generally require you prove the defamatory nature of the material - basically you need to bring your case before the judge to prove you have a reasonable cause of action against this poster. You should be prepared to present your "evidence" of the defamation (the libelous post made) and evidence of reputational injury caused by the defamatory posting. Without proof of reputational injury, you do not have good cause for a defamation action and the judge will not issue a subpoena and your defamation action against the poster will generally die there.
With proper evidence of defamation, however, you may also have the court issue an injunction against the poster at this time (this is similar to a cease and desist letter, although this order comes from the court) that orders him to stop the posting of any further objectionable material about you.
Courts are very protective of free speech, and are reluctant to issue orders prohibiting free expression without solid, reliable proof that the speech in question is defamatory or violates privacy laws or infringes in some way on another's legal rights. That is why it has become difficult to have material removed from an online website without prior proof of the material's illegal content.
