Speaking assertively is not magic. It is a technique that you can practice at home in your own time, by yourself or with close ones to provide feedback. This article provides some tips on how to go about assertive communication.
When you talk, use the word "I". Avoid using the word "you", as this stops you from trying to make the other person responsible, blame them or put words into their mouth.
Maintain eye contact in a way that is both natural and comfortable. Don't outstare a person but do look them in the eyes regularly to show your interest in them and your certainty in yourself. If you worry about looking into the eyes, look just above.
Keep your body posture alert and confident. Don't slouch and think of stretching yourself from your head to your toes, sitting or standing. Pay particular attention to the neck, shoulder and upper back regions. Also, don't give off angry or self-effacing vibes - stand/sit straight!

Avoid confusing messages. Mean what you say. If you say yes, you should mean it. If you say no, you should mean it. Don't go back on promises or firm statements.
Remember that silence is a tool, not an enemy. Learn to be comfortable with silence and use pauses to main effect to let the other person digest what you have just said. This is also useful for public speaking - people hang off what you have just said and wait for your next words.
Always know what you want when you are asking for something from another person. Keep this goal in mind and don't sway.
Speak clearly. Always. Mumbling, muttering and circular sentences do nothing to further communication.
Don't swear (curse) or talk rudely in tone. Use proper language.
Be careful about the tone of your voice. Keep it evenly moderated and avoid getting a high-pitched, whiny or tearful tone.
If you feel tears or anger coming on, breathe very deeply from the stomach - you should be able to see your stomach rise out and pull back in. This deep breathing will calm you in as little as 4 - 5 breaths.
Be careful to speak at a conversational level - not too loud and not too soft. If you are too soft, the other person will think you are trying to hide yourself and may ignore you. If you are too loud, the other person may become scared or angered by your voice.
The most important thing is to believe that you are responsible for yourself. No-one else is responsible for how you behave and who you are. And your wants are as real as anyone else's. Don't blame or resort to whining about another person's behavior.
Express your emotions clearly. Focus on how you feel rather than saying "you did X, Y, Z."

If you are someone who keeps your emotions repressed and your body muscles take the full brunt, it is important to exercise and stretch those muscles to stop this. You will stand taller, breathe better and feel stronger if you do this and exercise helps to free your emotions.
If the conversation becomes too heated, ask for time-out or a break. Explain that it has nothing to do with the other person, it is just that you feel confused, tired or need time to reflect and that you will resume the conversation later.
Don't yell or say things you'll regret later. This may seem like an easy option at the time but memories linger and it achieves nothing more than agitating and angering both parties. Assertiveness is about conveying your wants and needs without raising your voice.
Learning assertive communication takes time. Don't give up, just keep on practicing in everyday situations - supermarket queues are excellent!
Many of us try to seek people's approval in our actions, speech etc. Before we do something or say something, we try to think whether the other person will get angry at us or not. We try to avoid saying things that can earn the wrath of others, even if that is the right thing to say or do. When we let go of this habit, we will not only be able to freely express our thoughts, we can also become more assertive. we will say and do things without bothering what others will think of us, and it can help turn us into a more charismatic person.
You should care if you offend someone. You should care for others' feelings but in a positive way, not in a way which weakens you. There is no problem if you hesitate to say something because you do not want to hurt the listener, but if your hesitation is due to fear and not because of genuine sympathy for the listener, then you need to work on overcoming this negative trait within you.
If you have been in the habit of constantly seeking other's approval, then you might not overcome this habit overnight but if you keep working at it, eventually you should.
If you suffer any form of violence, seek immediate professional help or a refuge/shelter. Violence is not communication; it is domination.

Time to practice breathing and speaking clearly in front of a mirror
Family or friends you can trust to practice with
An open mind and a courageous heart