If Yakuza 6 is the first game apart of the Yakuza Series that you play, you probably don't know much about equipment and the effects it can have on Kiryu. Actually, the game itself doesn't even share the functions that equipment ultimately plays on Kiryu, as well as not properly telling players how to put equipment on the main character of the game! The creators put a lot into this game and even the series as a whole, but this is one thing they definitely forget to explain a little further.
While you explore the cities of Kamurocho and Onomici with Kiryu, you will ultimately collect loads of different types of equipment as rewards for doing various things like Substories. On top of this, should you have the desire to do so, you can even purchase equipment from the Ebisu Pawn Shop. Once you have equipment to wear, you will have to go into the Main Menu and select Stats. (Yes, that's right, there isn't even an Equipment Section on the Main Menu itself). From there, click on the Equipment Tab and then select what equipment to put on.
Even though you are likely to gain so much equipment during the Main Story, you will only be allowed to put two different items on Kiryu at any given time during the game. This might not seem like much, but it's highly recommended to do as soon as you have some equipment to do so with. Equipment can help boost your Stats - including your Attack, Defense and Heat Gauge - and they can even have additional effects depending on the type of equipment. This only can make a huge difference when you are in battle!