Negative thoughts are not reserved for just a few people or situations. Everyone is plagued by negative thoughts at one time or another.

However, once you begin thinking about what you're "thinking about", you've already taken the first step to controlling negative thoughts.

Make the decision that you're going to be the one to decide what your thoughts are and will be. You can control what you think about. This means making a daily effort to consciously program positive thoughts or affirmations into your mind.
Surround yourself with positive people. Have you ever heard the saying, "birds of a feather flock together"? It is the same with both positive and negative people. If your current list of friends does nothing but tear others down, complain, and generally be negative, it is going to be very difficult for you to get over being negative in that environment. You may, very well, need to make some new friends.
Refuse to accept negative stuff when you hear it from others. If you're making progress and you hear someone spilling out a bunch of negative stuff, simply tell yourself that it's not what you think, it's not what you believe, and it's not what you accept about yourself or anyone else.
Change your language. The words 'should', 'could', 'might', are out. For example change "I might go and fix the car" to "I WILL go and fix the car". That way you're talking action and making a promise you are more likely to stick to.
Think for yourself.
Use your initiative instead of waiting for people. You can waste a lot of time waiting for others to act.

Get fit. Having and sticking to an exercise plan can be very helpful.
Remember that actions speak louder then words. Rather than just talking, be willing to take some action.
Your life will go nowhere if you sit around doing nothing all day. Have some ambitions and goals and work toward them. Plan what you want to achieve in your day. Then make sure you follow through.

Not everyone is going to like that you want to be positive. For whatever reason, some people enjoy being negative, and you may find yourself fighting an ongoing battle for awhile until you make new friendships.