I'm not in your field, but generally you want to talk mainly about your research experience and your research interests. So you should discuss things like any research experience you had; what was it you did; how does your experience prepare you for grad school; what is it you're interested in researching in grad school; how you came about that interest and why you would be a good fit at the particular school you're applying to. Generally, you want to spend the majority of your SoP (even 60% or more) talking about what you want to do in the future, as opposed to what you did in the past.
Here's a blog post I wrote when I was writing my SoP: It's geared towards my field (Computer Science) but its still broadly applicable.
There's been a lot of really good advice given on this forum about writing SOPs and you can search the SOP forum for examples: http://forum.thegrad...tory-diversity/
You can also review examples of Political Science SOPs that have been posted here by successful applicants, such as and
Best of luck!