Your choice of words will always determine the quality of your life. There is nothing more powerful than your word. With a single word you can win someone's heart, or destroy a relationship you have worked so hard to build. Even more, a word can save a life or take it, start a war or end it.
For the meantime, let's just focus on the impact of words on relationships, because the quality of your life is heavily dependent on the quality of your relationships.
If I was only to choose one tip to give you about mastering your words it would be this: think for a few seconds before you speak.
I can at least think of a dozen times where I rushed into saying something that if I had thought about it for one more second, it wouldn't have left my mouth.
There are several factors you must always keep in mind before expressing an idea. For instance, you might have the right thing to say, but it is not the right time to say it.
Let me give you a recurrent example and try to see if you can relate to it.
Adam takes the time to cook for Eve and for some reason it is not exactly to her liking, and so she says "Oh, it's salty. I'll go make myself something else, but thank you".
As you read this, I am certain you saw what's wrong. Adam went through the trouble to make Eve a meal and she could not help but to make that comment and say no to his food. There is a clear lack of appreciation for Adam's efforts and absence of empathy in Eve's behavior.
I've seen it happen enough to say that there are indeed many people who do not know that even if something is true, there is a time and a place to say it, and that the other's emotions is something that must be considered.
Things would have been much better if Eve started with showing gratitude, not attaching it at the end of her discourse, and instead of stating her opinion on the food at that moment, she could have waited for later and told Adam in a much gentler way.
I can go on and on with more examples like this, but I think you get the idea: think before you speak, for what you say has a tangible impact on others.