I am a US citizen I want to invite my parents to come visit, what process do I need to follow?

It is unlikely that you will be able to have them come to visit. It would be better to just petition them for residency. Consulate Processing must be completed. This allows the petition to first be sent to the National Visa Center, and then the appropriate documents and package to go to the U.S. Consulate or Embassy. If done correctly, you will be able to enter as a Lawful Permanent Resident and will be in the U.S. in less than one year. Of course the time might be a bit less or more depending on the backlog of the U.S. Consulate or Embassy and whether or not a Waiver of Inadmissibility is needed.

You can write a letter of invitation and provide an affidavit of support along with supporting documentation of your income. However, your parents will need to qualify for the nonimmigrant visa by showing that they have significant and extensive ties to the home country warranting their return. There is no guarantee that they will be issued visitor's visas if they are not able to persuade the consular officer that they intend to return. In that case, you can always petition them for permanent residence (green card).