My first impression of the phrase "first entry" was "初めての投稿" (はじめての　とうこう). ２回目の投稿 (にかいめの　とうこう, my second entry) ３回目の投稿 (さんかいめの　とうこう, my third entry) Of course fairytale-san's answer "初めての書き込み” is also a good expression :)

初めての(first)書込み(entry)