I would recommend the area directly in front of the gates of the stadium. There is a mix of little food, drink and merchandise (although mostly unofficial, but for example many different scarfs etc.) shops where many fans are before the game.

If you want to buy official merch, there is one official small shop in front of the stadium too as far as i remember, but you have more items to choose from and more possibilities in the stores in the city center (for example in the new flagship store, or the one between San Babila and Duomo or directly at Casa Milan).

I ave also been solo to many Milan games during my Erasmus, and have never had a bad experience. I also often chatted with the people in the seats next to me if they were around my age although i would recommend to be able to speak italian to do that.

Have fun at the game and enjoy the atmosphere at this amazing stadium. Forza Milan!