Persona 5 uses a variety of social stats that allow you to interact with different systems in the world as well as increase your relationships with others. To raise your kindness, you have to be somewhat diligent, especially early on in the game. Kindness is used to progress your relationships with certain confidants.
Before anything else can be said about raising kindness, you need to clean your room and gain access to your plant as early as possible. Once you have access, buy some nutrients from the flower shop in the underground mall. Before you go to sleep, examine your plant and feed it. This can only be done once every couple of weeks, and it is a easy source of kindness. Check in with your plant every day and make sure that you always have nutrients on you.
Other than your plant, spending time with Sojiro is going to be your best bet. He is a crusty old man but sweet at heart. Melt his rude and bitter shell to increase your kindness. Working at either the flower shop or the Crossroads bar will also afford you ample opportunity for kindness.
Getting kindness up early is important, as you will not be able to increase your relationship with one of your earliest teammates, Ann, until you are at least level two. Do not let any opportunities pass you by.
You can also read books to increase social stats. You can check out books from the school library, or you can buy them at the bookstore. On the description for each book, it will let you know which stat will be increased by finishing it. You can read books in the school library or occasionally on the train.