I'll tell you how my experience went, maybe you'll have some ideas. I'm from Brazil, got a master's degree here, and I'm starting my PhD in the US next month (in a big state university, with a nice scholarship, in my opinion).
From what I saw, it is more about what you have accomplished so far, then what you want to specialize in, since there are programs on which you don't even choose an advisor until the end of your first year. This means you will have the time to get to know people and maybe change your mind along the way. Although, it can't hurt to look up the CVs and publications of professors you might want to work with, before choosing the universities to apply.
Anyway, I structured my SOP in four parts:
first: I started briefly telling my life history and how did I end up in mathematics. For example, during high school I took a two years technician course in Edifications/Civil Engineering, ended up screwing the entrance exams in the good universities here (because I focused only on the final project, which took 6 months), and was only accepted in a Licenciate course in mathematics (focused only in teaching). I ended up disliking working in Engineering, and disliking that math course. I was 18 at the time (I'm 24 now), and I wasn't aware that pure mathematics courses even existed (silly me, right?). But when I found out? Oh boy!
second: now I'm pursuing my B.Sc. degree. I had a fat scholarship, and I did a very long undergraduate thesis. My supervisor (which became my master's advisor later) suggested that we should revise it all, rewrite some things, add a topic or two, and throw in some exercises. Bottom line? I didn't even start my PhD, and our 560 pages book on introductory Lorentz Geometry (curves and surfaces) was accepted this month for publication with the Brazillian Mathematical Society. Go me! I went and finished my master's thesis in the same time we were finishing the book, ok.
third: if things were going so well, why did I want to leave? Basically, because the political and economical situation here in Brazil is crap (it definitely went downhill after the coup). I talked a bit about the budget cuts in science and education, and how I'd probably have to give up on mathematics if I stayed here.
fourth: if I decided to try going to the US, why did I choose to apply to this particular university? I concluded the essay with a personalized paragraph for each university.
Of course, what worked for me might not work for you, YMMV. You also have to consider other things in your application. I had a bad result in the GRE, but I got nice letters of recomendation (which were taken in consideration, despite my references not being world-class researchers). Also, even if the book (my main accomplishment so far) looks like a nuke, I only got accepted in one of the five universities I applied to.
I think that what you should take from my answer is the way that each part of the essay is connected with the previous one. That makes the text very fluid and the reading easier. I hope this is helpful and I wish you the best of luck.