Being agressive and being assertive are two entirely different things. An aggressive person gets things accomplished through the use of manipulation or force. An assertive person accomplishes things by honestly expressing his or her feelings and wants without the use of manipulation. Learn how to deal with people in a more assertive way!
Practice being assertive in the mirror at your home. Look into the mirror and roleplay being assertive with a co-worker, aggressive family member, stranger etc. Pretend you are trying to tell someone you do not like the way they treat you. Look into the mirror and say "I do not appreciate the way you treat me."
Keep practicing being assertive in the mirror. If you are dealing with peer pressure practice saying "I do not think it is a good idea to _______ (smoking, drugs, etc.) I am going to go on home."
Talk to your friends and family members about being assertive. Ask them what helps them to be more assertive. Start a conversation about assertiveness. You might be surprised how much you can learn from others about this subject!
Read additional information on the internet about how to be more assertive. Take assertiveness training workshops or classes.