Yes, perhaps you tend to be a negative person; some people are naturally negative, just as some people are naturally cheerful. But even if that's true, it doesn't excuse your behavior, nor should it keep you from changing (with God's help). You will be a happier person for it and so will the people around you!
What steps can you take? First, commit your life, including your tongue, to Christ. Ask Him to help you realize what you are about to say, so you can cut off any negative words before you say them. Let the Psalmist's prayer become yours: "Set a guard over my mouth, O Lord; keep watch over the door of my lips" (Psalm 141:3).
Then ask God to help you change your attitude. If you always focus on what's wrong, you will naturally become a complainer. But what if you begin to focus on the good things God brings into your life? When something good happens, stop and thank God for it and share it with others. I've never met a thankful person who was also a complainer!
In addition, make it your goal to encourage and help others by your words. The Bible says, "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).