Here's a brief summary of what you missed in Book 1 Kim Infected:
On Kim's eleventh birthday she received a letter from Hogwarts. She was the first one in her family to receive a letter because her three older siblings are all squibs, though her parents are wizards. Kim goes to Hogwarts and is sorted into Slytherin house, shortly after she is bitten by a werewolf. Kim keeps this a secret from everyone except her best friend Diamond who is super cool about it and wants to still be friends. Later they both befriend Cherokee and Leisel and they make up a group of four Slytherin girls who go around bulling people. (Which Kim isn't really into but kind of just goes along with what ever Cherokee says because Cherokee is cool and stuff.) Eventually Kim's friends find out that she's a werewolf and they're cool with that too but they've got this really mean headmistress who tries to make everyone hate werewolves and stuff.
On Christmas Kim goes home and her parents find out she's a werewolf and her grandmother gets really upset and says she can't live there anymore but yay for friends because they're smarter than the adults and know Kim can't help it. There's this new guy who comes over the break named Grey and he seems nice, he's super into fashion but Kim also knows he has a secret. Kim's friends try to become animagi but their potion fails and they are petrified. Kim runs to find her uncle Donovan who is also the defense against the dark arts teacher but he's not in his office. Suddenly Kim is attacked by Lilith, Grey's sister who does not go to the school and she grabs the first thing she sees to defend herself, unfortunately that thing is a portkey.
Kim finds herself in Azkaban Prison where she finds this guy named Marcus Waters who is really rich and has 12 daughters called the Althenian Princesses. He's trying to infect all the prisoners of azkaban with werewolf bites to create an army. Kim's headmistress appears and gets infected then Donovan appears and it turns out he's he son of the famous wizard merlin and has super powerful blood and stuff so he takes down Waters. Unfortunately Kim returns to Hogwarts before she sees that and runs to Professor Silva, Donovan's girlfriend who turns out to be spy. Donovan then proposes to her so Kim can't tell him that she's a spy because she doesn't want to ruin his happiness and she thinks she's probably not doing it of her own free will.
That about catches you up I believe. One more thing. Together Kim, Diamond, Cherokee and Leisel were able to conjure a corpeal patronus once when they were all in the forrest and curiously enough they resemble some people you might recognize but I mean, I don't know, it could just be a total coincidence: Cherokee is a stag, Diamond-grim, Leisel-rat and Kim, well a wolf. Just thought I might include that :)
There we go! I hope you all enjoy the sequel to Kim Infected!