If you are not aware of your thoughts then it will be difficult for you to control your thoughts. Rather your thoughts will control you.
Most of the time we just think by default. We are not aware of our thoughts. We are not aware of which thought is coming and which thought is going; and how one thought is connecting to another related thought.
So, how to control thoughts?
To control your thoughts, first you have to raise awareness within you. Once you raise awareness within yourself, you become like an observer. You can observe what is going on inside of your body and mind. You can observe which thought is coming and which thought is going.
As an observer you can also consciously choose your thoughts. You can consciously choose the thought you want and reject the thought you don’t want.
Note: So, raise your awareness first. And to raise awareness, you can practice meditation. You can practice mindfulness.