Rash guard or a t-shirt?

Start in a t-shirt. Nobody is checking, and I am not going to make you spend money before you know you like this.

A rash guard is better once you are grappling, and for three boring reasons. It stays tucked, so you are not pulling your shirt down off your ribs every thirty seconds. It saves your skin on the mat. And it gives your partner nothing to grab, which matters in no-gi, where a fistful of cotton is the cheap way to control someone.

Cotton also holds sweat and smell in a way that gets unpleasant fast. Whatever you wear, wash it after every session.

If you are buying one anyway, buy long sleeve. More skin covered, fewer mat burns. The full kit list is short, and none of it is urgent before your first roll or before you decide whether the gi matters to you.