Positive thinking is a constructive approach to life and its challenges. Positive thinking is not (as it is popularly misconceived) saying " I can do it" when a task is beyond a capoability. Negative thinking is a destructive approach to life's challenges. Negative thinking is saying "I cannot do this" when you actually can.
Negative self-talk can take many forms:
Polarizing: You see things only as either good or bad. There is no middle ground. You feel that you have to be perfect or you're a total failure. E.g., "I have to do things perfectly, because anything less than perfect is a failure."
Filtering: Here you magnify the negative aspects of a situation and filter out all of the positive ones. E.g., on a day when you completed your tasks ahead of time and were complimented for doing a speedy and thorough job, you think only about all the rest of the tasks you have to do and how difficult they are, instead of what you have achieved and the compliments you received.
Personalizing: You automatically blame yourself for something that went wrong. E.g., Your friend cancels an evening out, and you immediately assume that the change in plans is because he/she did not want to be with you.
Catastrophizing: You automatically anticipate the worst. E.g.,The reaturant gets your take-away order wrong and you start cursing and thinking that the rest of your day will be a disaster.
Negative Self-Labeling: "I feel like a failure. I'm flawed. If people knew the real me, they wouldn't like me."
Excessive Need for Approval: "I can only be happy if people like me. If someone is upset, it's probably my fault."
Pessimism. "Life is a struggle. I don't think we are meant to be happy. I don't trust people who are happy. If something good happens in my life, I usually have to pay for it with something bad."
You can learn to turn identify your negative thoughts and turn them into positive thinking. The process is simple. However it takes time and practice because you first have to break an old habit, and then create a new habit. Here are some ways in which you can do it.
First identify the areas to change: Identify areas of your life that you typically think negatively about, whether it's work, your daily chores or a relationship and list them out.
Start small: Choose just one area to approach in a more positive way.
Identify the thought and practice changing it: Periodically during the day, stop and evaluate what you're thinking. If you find that your target negative thought is cropping up, try to find a way to put a positive spin on it . A few examples: are given on the right.
Change "I have to do things perfectly" to "Let me just finish the job"
Change "Oh God; I have so much to do" to "I just finished a task and it went of well. Let me see what to do next".
Change "I feel like a failure. I'm flawed. If people knew the real me, they wouldn't like me." to " No one is perfect. I am me. And people will have to like me for what I am".
Change "Something is going to go wrong", to "Let me do this and see what happens. If something goes wromg I will deal with it hwen it happens".
Change "Life is a struggle", " Life is full of challenges, let me deal with this issue first, even though it is tough". 
Change "I've never done it before". to "It's an opportunity to learn something new".
Change, "It's too complicated", to "Let me tackle it from a different angle".
Change "I don't have the resources" to "Necessity is the mother of invention".
Change "I was unable to do it" to "I wasn't able to fit it into my schedule, but I can re-examine some priorities".
Change "There's no way it will work". to "I can try to make it work".
Change "It's too much of a change" to "Let me take a chance and try it out".
Change "No one bothers to communicate with me" to "Let me see how I can open the channels of communication".
Change "I'm not going to get any better at this" "Let me give it another try".
Once you have suceeded is changing the target negative thought, move on to the next one on the list. 
Don't expect to become an optimist overnight. With practice, eventually your self-talk will contain less self-criticism and more self-acceptance. You may also become less critical of the world around you. When your state of mind is generally optimistic, you're better able to handle everyday stress in a more constructive way.