The first step to positive thinking is recognizing your negative emotions.
For this you need to ask and answer questions on what things you do not like and why? Now, without reacting to the feelings, slow down and see why you feel that way and how you can change them. Most often the answer would be patience and practice.
To nurture positive thoughts and replace negative ones with them would mean identifying the negative thought, debating its presence and seeing which positive thought will fit the bill. Following these steps will soon convert the process to a habit.
Eventually, positive thinking will become a habit.