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. Charm is gained in a few different ways and mostly helps you increase your relationships with certain confidants.
To consistently raise your charm, go to the bathhouse right outside of Cafe Leblanc. Attendance costs some Yen and passes the time, but you will come out squeaky clean and full of … charm? Why not? Going on Mondays or Thursdays will grant you bonus charm as well. If it is raining, head to the bathhouse for a guaranteed boost regardless of the day.
The bathhouse is the least obscure way of getting charm. Other ways will pop up periodically, such as excelling on your exams, spending time with certain confidants, eating at out certain restaurants or working in the Crossroads bar.
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.