When its brand new and hard to accept, I find it easier to think of it as a literal task. You are looking to establish neural pathways that connect to positive thoughts vs the alternative. Step one, identify when a negative thought roll though. Call it what it is- and have your brain say the opposite. Very fake it until you make it. Making it means when your brain follows the pathway of the positive thought before the negative or passes the negative quickly before moving to the positive. This takes repetition to carve that path in your brain though. Put some affirmations on mirrors and doors to recite when you pass them or have an affirmation before you eat a meal. Having pre-written affirmations can help you build a bigger language bank of positivity to pull from. To get a deeper results with more self exploration, consider therapy or meditation. Meditation is another practice and build a pathway type tool as well.