Fluff vs Angst: It could be argued that fics can be separated into two main categories: fluff and angst. Fluff fics are generally happy, cute little stories with very little sadness or death. Angst fics are the opposite, often dealing with dark themes or featuring arguments, violence, or character death. Of course, many fics mix these two elements, with characters going through periods of turmoil and pain to arrive at a fluffy happy ending. It's important to know whether a fic is mainly fluffy or angsty before you read. There's nothing worse than going in expecting a happy love story only to have everyone die at the end.
MCD, CTV, DubCon/NonCon, and other warnings: Fics, especially those on AO3, often come with content warnings. These warnings allow readers to know what they are getting themselves into before they begin reading. Since these can be important trigger warnings, I want to go over some of the more important ones in depth here.
Major Character Death (sometimes abbreviated MCD), means that one of the major characters dies. So, if you don't want to read a fic where your favorite character gets killed off, skip this. Authors will often tag minor character death as well.
Canon Typical Violence (sometimes abbreviated CTV) means that a fic contains the same kind of violence that is typically seen in canon. So, while in a Harry Potter fic this may only mean a minute or two of the Cruciatus curse, in a Walking Dead fic this may mean extreme gore and violence.
DubCon and NonCon both refer to the sexual content in a fic. DubCon means dubious consent, while NonCon refers to clearly non-consensual sexual acts. For obvious reasons, this kind of content is nearly always clearly tagged with multiple warnings.
For your own mental and emotional safety, please read all tags and warnings carefully before starting a fic. Now that you know what kind of content to expect from a fic, let's go over some common tropes.
Tropes and Common AUs
Fake!Married or other pretend relationships: For some reason or another, the characters have to pretend to be in a relationship for a period of time. Maybe they need cover for a case, maybe they want their family to quit nagging them, maybe they don't want to go to a wedding without a date. For whatever reason they pretend to be in a relationship, and somewhere along the way things get complicated. This is a perennial favorite fluff fic trope. It's just as cheesy as it sounds, and I love every second of it.
Coffee Shop AU: These fics take place in a coffee shop. Maybe two characters work there together, or maybe one is a barrista and the other is a regular customer. Either way, it's a perfect excuse for characters to meet and interact regularly.
SoulMate!AU or SoulMate!Verse: These fics take place in an alternate universe where soul mates are a fact of life. There are a couple common types of SoulMate!AU. Characters may have counters on their wrists that count down until they meet their soul mate. They might have their soul mate's name on their wrist, or the first words their soul mate says to them. They might only see in black and white until they touch their soul mate, when the world suddenly bursts into color. No matter the details, you can imagine the possibilities.
Genderswap: One or more of the characters are a different gender than they are in canon. There are two types of generswap fics: ones in which the character has always been that gender, and ones in which the character changes genders during the fic, often due to a magical spell or curse.
High School AU (HS!AU): All the characters are kids in a regular high school. Is your favorite character a jock, nerd, band kid, or something else entirely? Do they do well in classes? This is an incredibly popular AU no matter the fandom.
Hogwarts AU: Imagine all your favorite characters from another fandom going to Hogwarts. They take classes, learn magic, have adventures, and put up with professor Snape. These fics usually take place at a different time than the events of the HP series, and therefore do not usually feature Harry and the gang. Sometimes I think half of the fun of these stories is sorting characters into the proper Hogwarts houses.
Everyone Lives AU: A canon-verse fic in which no-ones favorite characters have been killed off. Usually these fics take place in the canon universe, but ignore the deaths of characters for various reasons. Hey, they might be dead in canon, but they live on in our hearts.
Well, now you know pretty much everything you need to know to find the kind of fanfiction that will make you happy. No matter what you like, what fandom you're in, and what you ship, there is a fic out there that is perfect for you. Go forth, friends, and read!