Transforming into Sailor Moon
It was given to Usagi by Luna in episode 1 of the anime and Act 1 of the manga. Luna bribed her into transforming into Sailor Moon (tricked her in the manga and crystal) in order to prove to Usagi that she had a destiny as a Sailor Senshi. Once she obtained it, Usagi began wearing the brooch in the center of the bow on the front of her school uniform when she was in civilian form.
The brooch was broken in episode 51 of the anime and Act 14 of the manga, but received the Crystal Star afterwards, which was far stronger and more powerful. Starting with the Crystal Star, all of Usagi's other evolved compacts were so much stronger than the others and had even more powerful weapons.