Every word you say will have an affect on others. Positive words will encourage and strengthen other people, but negative words will tear down and harm them. You must decide whether to speak positively or negatively. Please understand that speaking positively requires a deliberate decision, whereas speaking negatively comes easily—for it is human nature. Therefore, actively strive to speak positive and encouraging words to others.
It is no surprise that the Bible instructs us to say beneficial things about others: "Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen" (Ephesians 4:29). Furthermore, Colossians 4:6 says, "Let your conversation be always full of grace, seasoned with salt ..." Speaking positive, uplifting words is extremely important. Indeed, Proverbs 25:11 says, "A word aptly spoken is like apples of gold in settings of silver."
In Psalm 19:14, David prays, "May the words of my mouth and the meditation of my heart be pleasing in your sight, O Lord, my Rock and my Redeemer." David clearly knew the importance of words, and so he prayed that he would speak words that were pleasing to God.
We must deliberately choose to speak positive words; otherwise, we will instinctively complain and be negative—for that is human nature. Thus, it is no surprise that Ecclesiastes 5:2 tells us, "Do not be quick with your mouth, do not be hasty in your heart to utter anything before God. God is in heaven and you are on earth, so let your words be few." Similarly, Proverbs 10:19 says, "When words are many, sin is not absent, but he who holds his tongue is wise." Usually when you "hold" your tongue, you'll catch yourself before you say nasty things about others and cut them down with your words.
Lastly, it is important to understand that whatever you say is a reflection of your heart attitude. Matthew 12:34 says, "For out of the overflow of the heart the mouth speaks" (also see Luke 6:45). In other words, your words are simply an overflow of what's in your heart. If you think poorly about others, you'll speak poorly about them. On the other hand, if you think positively about others, as the Bible instructs us to, then you'll speak positively about them.
Since words carry power, make a decision to use your words as building blocks not battering rams.