Lucy,
This is a huge question. I suspect you will get answers from a lot of different directions. Here's my take:
The easiest and most effective way to improve your communication skills and to excel in debate is to listen. Listening is a lost art, not well practiced in our society, but if you learn to listen intently and actively, your communication ability will increase. You might consider reading Covey's "7 Habits" book which touches on this.
The idea of standing up for yourself is a different topic. Consider the circumstances (when, why, what's going on) of some occasions when you did not stand up for yourself. See if you can figure out what the problem is. You may need help on this--consider finding a local Licensed Professional Counselor (search for Licensed Professional Counselor 12345 where 12345 is your ZIP code).
I'm certain that my answer is not complete because your question is so big, so hopefully others can add some good ideas as well.
