AP US Government and Politics
In comparison to many of the other tests, the AP US Government test is not very hard. If you watch the news/stay informed about life, you should be able to answer at least 40% of the questions already.
If you are planning on self-studying Government, it would be very helpful to have taken a US History class, and perhaps European History (not so much Euro, but definitely US History). Euro would help you with the Enlightenment thinkers and what the constitution was really born out of, but US History really helps with all of the acts and amendments that you need to know about.
As far as prep work goes, I used the Princeton Review AP Government book, I thought it was fairly helpful. It was a little bit shallow and lacked information at times, but it tells you what you need to know. I really liked the practice tests though. I felt like they were very similar to the real thing and were quite helpful when it came time to practice.
Otherwise, I also used the REA Crash Course to AP Government which was beyond amazing. I cannot sing my praises loud enough. I absolutely love the REA Crash Course books! Try not to rely on them as your only study method though. They will often times help you rake up multiple choice questions if you read the book right before your test. I think the best part comes at the end, the book lists the top thirty court cases, acts, etc. that you need to know for the test. So in other words, there is a crash course inside the crash course.
Just a final word of advice, don't stress out about the free-response questions on AP Gov. This is probably the only test that they give you way more time than you actually need, but at the same time, I caution you to actually write coherently and well, and most of all make sure you answer every single part of the question!