Back before the first anime was made, the manga was pretty new and was getting really popular, so they decided to animate it. But the problem was that it was only about 1/4 of the way through. So instead of waiting on it to finish and risk it becoming unpopular in that time, they decided to use what they had of the manga and then split off with their own story that was different from the manga's after that. Years passed, and the manga still remained really popular, so they decided to go back and create a new series that's faithful to it. And thus, Fullmetal Alchemist: Brotherhood came to be. It's only similar to the first anime for about the first thirteen episodes or so before it leaves that plot almost completely for stuff that was never in the first one.
You don't have to watch the first series to understand Brotherhood, because they do speed through all the relevent information that was covered in the first anime. I do recommend that you watch the first anime at some point, because it's still a really good show, but it isn't required to follow the new one. Skip the movie though, because that thing just sucked royally.
I don't know what's real from the noise, the noise.
I can't hear a thing for the noise, the noise.
I wouldn't do anything, the thunder that throws me away.
I wouldn't do anything, there's something that keeps me away.
I wouldn't do anything, something that begs me to stay.
I wouldn't do anything, something that throws me away.