There's a world of difference between being assertive and being arrogant. Very few people like someone who's arrogant, but being assertive means that you get your opinion across to others, but in a pleasant and friendly manner.
Much is written about being assertive, and I suggest you start here:
http://www.wikihow.com/Be-Assertive
From this rough introduction, use Google to find out more information. What I find particularly useful is how to say No when I'm asked to do something I don't want to do. It's the most powerful word in English.