First of all, it should be noted that Mesagoza is a pretty darn big city, but thankfully it isn’t too difficult to get to your destination: the Academy! All you need to do is just head directly north from the entrance and you’ll get to where you need to be, don’t worry.
However, there are a ton of items located around Mesagoza if you want to pick them up, and so many shops that you’ll probably be overwhelmed on your first few visits.
Don’t worry too much about buying anything; you don’t really need any of the food from restaurants just yet, and while you can stock up on some supplies for making sandwiches at a few of the shops, it isn’t really useful at the moment. There are two Pokémon Centers in the city: one in the western side of the city and one in the eastern side. Keep that in mind, as you’ll often want to fly back to one of them while returning to Mesagoza in the future.
Team Star is Recruiting
As you approach the Academy, climbing up the steps near the center part of the city, you’ll find a girl with an Eevee backpack get approached by some troublemakers wearing white helmets and star-shaped sunglasses trying to get her to join Team Star. You can either offer to help her out or walk away—be sure to help her out, because she needs some help, although that will pit you in a fight against a Team Star Grunt.
She uses a Shroodle, which is a Normal/Poison-type and shouldn’t be that bad to fight, though watch out for any of your Grass- or Fairy-types.
Afterwards, Nemona will arrive and recognize that group as a bunch of deliquents, and then she’ll give you something useful: the Tera Orb. With the Tera Orb, now you’ll be able to Terastallize in battle, changing your Pokémon’s type temporarily to its Tera Type and powering up its moves of that type, as explained earlier.
Even if a Pokémon has two types, after Terastallizing, it will only be of the type that matches its Tera Type, and for most Pokémon caught in the wild, their Tera Type will be one of their types, chosen at random. This can make Terastallizing a good defensive strategy as well as an offensive strategy, by Terastallizing strategically to get rid of one of its weaknesses.
Unfortunately, you can only Terastallize once per battle, and after Terastallizing, you’ll have to visit a Pokémon Center to heal up before you’ll be able to Terastallize in most instances (there are a few instances where you don’t have to heal, such as way later the game, while fighting the Elite Four, for instance). That means you shouldn’t use it all the time.
Anyway, now you’ll get a chance to see it in action by fighting the other Team Star Grunt.
You can press the R button while selecting a Pokémon’s moves to Terastallize it. Go ahead and do that in this fight if you want, although it isn’t necessary.
Now that Team Star has been scared off, the girl you were protecting earlier will thank you for the help and then run off, leaving you free to head up the stairs to the Academy, which is your next destination (unless you want to explore more of the upper area of town).
Optional: Visit the Pokémon League Early
There’s a tunnel in the northwest part of Mesagoza, north of the western Pokémon Center, that leads you to the outside area of the Pokémon League. You’ll obviously need to come here towards the end of the game, but you can visit it now to pick up a few items and find some Pokémon. There are some decent Pokémon you can find here early, such as Tandemaus, so it might be worth checking out.
Just beware the Trainers in the area: one of the them near the entrance isn’t too tough, but the one near the Pokémon Center is extremely strong if you fight him at this point in the game, using Pokémon over level 50!