Start by paying attention to what you think about over the course of the day. What do you worry about? What do you dream about? If it takes up mental space, you care about it.
Identify the top 3 things that you think about throughout the day. After you do that, think about whether those 3 things are in fact the 3 things that you want to spend the most mental energy on.
Mental energy does not go on forever. Like everything, it has its limits. It's why Steve Jobs decided to wear the same thing every day - he didn't want to waste mental energy on making decisions about what to wear. He decided to reserve that mental energy for more important decisions throughout his day.
Chances are that the top 3 things that you are thinking about do not match what you want to spend the most mental energy on.
Confront/address one of the 3 main issues that you think about so that you spend less mental energy on it. For example, maybe you spend time worrying about that kitchen door that needs to get fixed or that tough question that you have for a significant other or that paper that you want to start writing. Call the handyman, ask the question, start writing. By addressing the issue that weighs on your mind, you will have more mental energy to care about things that you actually want to care about.