Her name was Serena. One day she saw some little brat children picking on a cat so she chased them away. She picked up the cat and saw a band-aid on its head. She pulled the band-aid, and there on its forehead was a crescent moon. The cat fretted its way loose of Serena, and Serena, whining, scurried off towards school, late again.
And failed her test. She was so totally bummed although she knew it was her fault (although she didn't admit it) and to cheer her up her best friend, Molly, suggested that they go shopping at her mother's store. Molly's mother was a jeweler, so Serena agreed.
Meanwhile trouble was brewing, although Serena didn't know the meaning of the word trouble yet. Well, she did, just not in this context. She might be a meatball head, but she ain't that dumb, sheesh...
Anyway, the Nega-verse was plotting to take over out Universe, and Queen Beryle, the evilest (well, at the time) of the Nega-verse citizens decided to attack our universe to seize control of it. She summoned her best warrior Jedite (again, at the time) and he set off on a course of world domination.
Or universe domination, I guess...
To this end, he sent one of his minions to Molly's mother's shop, and this minnion slashed prices drastically. Now besides causing Molly's mother to loose all her hard earned cash, this meant that the minion could infect all the jewels with a curse of sorts which would drain all their energy so that it could be given to the Nega-verse so that the Nega-verse could break free. Since prices were slashed to near nothing, everyone had to have one of these jewels.
Now Serena and Molly thought this was just slightly weird. They tried to shop, but in the midday rush were unable to purchase life-draining trinkets of their own. Serena, bummed, went on her way home, showed her mother her bad grade, and suffered the wrath of her mother.
Now, as for the cat (remember the cat?) the cat was trying to fight the Nega-verse, but really couldn't do it on her own. She was looking for a mighty warrior, and although she could feel something coming from Serena, she could hardly believe that the fate of the world could be entrusted to this whining brat... but the more she looked at Serena, the more it became obvious.
So the cat, Luna, approached Serena, and told her of her destiny. Now, Serena was just a little upset (hey, how would you act if your cat talked to you?) and couldn't believe this was happening. Luna pressed on and gave her the whole story including a locket which she could use to become the warrior Sailor Moon.
Serena continued her protests and finally Luna said, "Now say, 'Moon Prism Power!'" and Serana said it and transformed into Sailor Moon.
Now this is where we get into the logic of the animation world. Sailor Moon and Serena are basicly the person, they look the same, they have the same voice, and they're both klutzes. They differ in two basically non-important ways. One, Sailor Moon's costume is slightly different. The skirt gets a lot shorter, her shirt changes color, her meatballs on her head get little ornaments and she gets a tiara and a choker. Also Sailor Moon says things like, "I stand for love, and I also stand for justice," and, "In the name of the moon I will punish you," and "You're moon dust."
So anyway, Sailor Moon heard Molly yelling in terror (using a power which was only to be seen once in the whole series) and rushed the rescue. There was Molly, being attacked a monster.
When she got there, she discovered one important detail. She didn't know how to fight. Fortunately she had Luna to tell her to yell out, "Moon tiara magic" and when she did, the monster was destroyed.
And thusly starts Sailor Moon.