My new year's resolution for the year 2019 was that I will learn to be more assertive. It means that in every situation I'll try to create my own opinion and tell it to other people, but still do it without fighting against their thoughts. Accepting other people and their opinions is also part of being assertive!
Firstly, you have to create an opinion, and you have to be sure that you can explain it to others too. Explaining is very important, because by doing that you avoid negative comments and you base your opinion. That also leaves no doubts for others that you are really trying to say something.
Usually people have their things to say too, so you have to be ready for protest. And here we get to the other important thing: staying on your opinion. This is something you just have to learn, and in the beginning it might be really hard to stay on your own opinion.
Anyway, the most important thing you have to remember is that you don't harm your friends or other people you're arguing with. Because being aggressive won't help anyone - you have to create an opinion, learn how to stay on it and appreciate others opinions and things that are important to them.
And THAT is the key to assertive behaviour.