...But you'll come across "Fullmetal Alchemist" AND "Fullemtal Alchemist: Brotherhood". WHAT DO I DO?
Fear not, for I am here to aid!
FULLMETAL ALCHEMIST is a manga created by Hiromu Arakawa. It started back in 2000-2001 ~ish, and an anime adaptation was made by Bones as of 2003-2004 ~ish.
However, at that point, the manga itself was far from over; because of that, Arakawa offered the material she had thus far, and instructed the production team in Fullmetal Alchemist's world. She told them to make up the story past the point where the manga was at that time, however (And apparently it was rather early on). So from about half the 2003 Fullmetal Alchemist anime onwards, the story follows a completely different direction. Keep that in mind: most of 2003 FMA isn't technically canon, since it's not a manga adaptation, but something created by Bones.
Way into 2010, however, the manga series was almost over and FMA: BROTHERHOOD started being produced. Now THIS is the actual manga adaptation; the whole canon is here. While you should not disregard the original 2003 series, Brotherhood is the actual source stuff, since it follows the manga strictly (It goes over a few details, but nothing that will ever hinder the understanding of the plot and the fun; minor things, mostly). So if Brotherhood's plot and story contradicts 2003's FMA, take Brotherhood as rule, since 2003's FMA wasn't actually done by Arakawa.
"So what should I do now?" Watch BOTH. But start by the 2003 series. Seriously, just do it. There are two reasons for that and I'll explain the second one later.
The first one is that it gets you introducted to the FMA universe, the characters, their personalities, and the overall feel for the series. It's fun, it's hilarious, it's sad, it has "The Feels" moments, and it's simply a wonder to watch, Getting started by 2003's will cause you to love FMA, but you're only getting a tiny part of the fun.
With 2003 FMA watched, you may decide to watch Conqueror of Shamballa, the movie sequel to 2003's plot, but I just jumped straight into Brotherhood. Its up to you, but Shamballa is also not Arakawa-canon.
The second reason is that the first episode of Brotherhood is actually not entirely canon. It's NOT the same as in the manga, and not the same as the 2003 series. Why is that? Think about it - how fun would it be to watch the SAME episodes that started the first series in order? In 2003's and the manga, the starting point is the Elrics' little trip to Leto. In Brotherhood, that only happens in the third episode or something. That's because not only it would be annoying to have the first episode to be the very same than that of the last anime, but it would cause mass confusion, since both begin similarly but are way different. This way, the fuss is lessened.
They just made something up and tucked in as the first episode to re-introduce the world and the characters. From the second episode onwards, it's pure manga canon. If Brotherhood tells you something that contradicts 2003 FMA's teachings - listen to Brotherhood. Don't disregard 2003 at all, but keep it as an introduction to FMA's universe before the actual fun begins. Trust me, watching 2003's, then Brotherhood is the way to go, always.
Some characters change, some characters don't exist in the opposing version, but they're both fun. In my opinion, Brotherhood > 2003 FMA, but watching 2003 FMA just makes Brotherhood much more fun!
If you'd like to discuss more with me, just lob me a message. I'll explain more in-depth, but just have fun with the series. Aaaaand don't say anything about Ed's height. Ever. He gets so pissed.
I would need to get list and read all of these, sometime.
