Fanfic is fun! It's like writing with stabilisers.
Probably the best place to post is Archive of our Own, which is a massive site, quite easy to use, and covers pretty much any book/film possible. Things like Fanfictionet are a nightmare to post to (formatting etc).
Some acronyms are specific to a particular series, e.g. EWE in Harry Potter means 'Ending, what ending?' and means that the whole Deathly Hallows epilogue is ignored.
PWP means, er, porn without plot, or plot, what plot?
There are various acronyms for reader age, and there may be guidance as to what should be listed as 12, 15, 18 etc... Play it safe, you don't want little kids stumbling across your PWP!
A 'ship' is a relationship and those in the relationship are usually marked e.g. harry/ginny.
Slash is where the writer does a relationship between two male characters who are (generally) not in a relationship in the canon (original material).
RPF is real person fic, and some sites won't take it as it leaves them open to legal action.
AU = alternate universe, imagine Harry as a muggle wizarding policeman.
OOC = out of character, and is where a known person/character is made to act in ways that don't fit at all with their canon character. Hard to pull off.
OC = original character. If you're reading fanfic then a few original minor characters is ok, even good, but a lead OC is often a 'mary sue', a fictionalised version of the writer in idealised form. Often excruciatingly bad writing. e.g. Harry leaves Ginny and whilst on the run meets an unbelievable gorgeous teenager called Alice, which just happens to be the writer's name, and she's very clever, has super powers, and Harry falls in love with her. Wish fulfilment, basically.