To be able to say yes or no, first you have to be clear what you want. Take your time to decide what kind of relationship you want – with a partner, a friend, at work.
After you learn to be assertive, don’t be afraid to express your desires. Everyone has the same rights.
Finally, learn to respect the will of the other when they say “No, I don’t love it, it doesn’t appeal to me, I don’t like that…”