If someone has posted very defamatory comments about me online, how do I proceed with legal action?

Review the situation with a competent attorney. Don't assume that the comments meet the legal standard of defamation but, if they do, you may have remedies against the poster and/or website perhaps even criminal ones.

You will have to file suit. Ask your local bar assn for a referral to a lawyer who handles such cases.

Write or e-mail the person, cc to the web site, and tell them you have suffered damages, the comments are clearly false, and if they are not removed within the next 3 business days you will file suit for all of your damages along with punitive damages, and that you will include the web-site for publishing information that it should know is false and defamatory. State that your attorney advised you to send this demand letter before he files suit. Try to post on the web-site a statement that the information is false, published to hurt you, you can be contacted to tell the reader how it is false, and you will be filing suit. Then go to local attorneys who handles such suits and find out their recommendations; initial interviews are normally at no charge. Try to see if the offended person has any assets worth going after.