How is a divorce initiated?
A divorce is formally initiated when one spouse or the other files a form (usually called a 'Petition For Dissolution') in the Family Court in your jurisdiction.
The attorney for the filing, or 'moving' party collects information from that person regarding the marital estate, minor children (if any), debts, assets, etc, and enters it into the Petition For Dissolution. The Petition is sent to the Court Clerk for entry into the Family Court list of pending cases.
At the same time, copies of the Petition are given to you, generally by a process server or a "disinterested third party". At this point you must respond within a certain amount of time (typically3 to 4 weeks) or you lose your right to have any input into later proceedings.