Hollows are creatures in Bleach that reside in Hueco Mundo. They are born when human souls fail to reach Soul Society and instead remain in the human world and become corrupted. These spirits then begin to consume the living and dead souls of other humans. As such, it only makes sense that if you play as one of them in this game, it forces you to do the same. So if you want to become a Menos, you must eat Hollows in Type Soul. The more you devour, the stronger you become. However, if you're not sure how that works, here's a quick tutorial to help you out.

You can eat other Hollows by pressing B while they have their dead bodies.

Now you have two options: you can continue defeating other Hollows, or you can move on to Adjuchas, followed by Vastolorde.

Alternatively, you can press CTRL + K to tear off your mask and become an Arrancar. While it is recommended not to do this right away, the later you take off your mask the better, as it will make you a stronger Arrancar.