Ok, so I've decided to start watching FMA Brotherhood. However, I haven't watched a single episode of FMA. Is it necessary to watch FMA before FMA Brotherhood or can I just skip it?
Brotherhood is the version that follows the manga, it got a more hectic pace and it's ranked No.1 on MAL.
That being said, FMA remain great in its own merit, though it does not got the above mentioned factor and it's recommended to watch The Conqueror of Shamballa after the series, to get a true conclusion.
So, yeah, you may watch Brotherhood directly but FMA remain a great anime. There's less than 1 point difference between the two if we were to rate them.
Both of them are simply alternate versions of the FMA tale. Fullmetal Alchemist starts off close to the original story but eventually diverges, while Brotherhood sticks much more closely to the manga tale. There is no need to watch one before the other.
That said, I personally would say Brotherhood is by far the superior story and makes the original pointless. It's less that the original was bad but the first FMA anime is nowhere near as good as the manga and is pretty underwhelming in comparison. I gave the first anime a 7/10, while Brotherhood was a 9/10 for me and the manga was one of my few 10/10s. First FMA's pacing is pretty laggy and there's a lot less depth to the story and characters by comparison.
original FMA has a darker, deeper feel to it. The way Brotherhood is done makes it feel like its aimed at younger audiences. Gave me the impression of Dragonball Kai.
FMA follows the manga 1/3 of the way and becomes it's own storyline. And, it ends up being much darker, deeper and maturer.
And FMA (B) follows the manga in every way and becomes either one of the best or the greatest anime ever to be made.
As to what you should watch. You should watch that FMA first since it has a sad ending and then watch FMA (B) so you can be happy at the end and know the actual futures of both Al & Ed.
P.S. After you watch FMA, watch "Conqueror of Shamballa" it's a movie which ends the animes storyline.