It's a difficult balance, standing up for yourself, your views and your ideas without coming across as angry or hot-headed. I had the opposite problem. I was SO passive growing up, it wasn't even funny! I would always get stepped on and walked all over. I could never stand up for myself.
It's easier said than done, and best often learned through experience. Sounds silly, but this could me my experience working in theater and a degree in communications talking here, but ROLEPLAY. It's often not what you say, but how you say it. Certain situations are difficult, even as an adult. Roleplay, roleplay, roleplay. Actions, words, intonation, etc. It all matters when you communicate with others.
Look at the politicians on television. No one specific, just in general. There are many of them who get their points across well, and others who fumble and get angry and talk in circles. Like I said, it is NOT easy to communicate. Especially at 11! So many kids are so childish and silly, and others mature. It's also a hard age group too, usually late elementary/first year middle school and things are hard when it comes to routine and making new friends.
Like others said, try and think about why she is argumentative and take it from there. It's okay to be opinionated and vocal, but you have to do it the right way. It gets easier as you age, as you probably know. But telling her that now may go over her head.
Best of luck!