Short answers, pulled from the story.
The Japanese Navy General Staff first considered Operation Mo in 1938. This early planning phase focused on consolidating control over Southern Seas areas during the late 1930s.
On the 3rd of May, a Light Task Force occupied the port of Tulagi near Guadalcanal in the Solomon Islands. The same unit also took Nauru and Banaba Ocean Island for their valuable phosphate deposits.
Rear Admiral Kiyohide Shima led the Tulagi assault force with a Minelayer-cruiser and Seaplane tender. Rear Admiral Sadamichi Kajioka commanded the Port Moresby occupation force with a Light cruiser and multiple destroyers.
USN codebreaker Rudy Fabian at FRUMEL briefed General MacArthur who found this information incredulous. Fabian explained the JN-25 code-breaking process to show him the Japanese commander's intercepted objectives.
An Australian reconnaissance plane discovered the Port Moresby invasion force on the 5th of May. This discovery led directly to the Battle of the Coral Sea which prevented the Japanese landings against Port Moresby by sea.