Spoilers ahead.

First of all, we are extremely well introduced to it world. The first scene shows a huge Titan sticking out the wall, while the show explain to us what the humanity knows about the Titans, that is, particularly, not much. The only think we know about the humanity, is that they live in 3 different walls, and they have been in peace for hole century, but no one even know how the walls were built or even what had happened before.
After that scene, which don´t really know if it´s a moment from the past or future, we are teleported to our 2 of the 3 main characters, Eren and Mikasa. Later on, they will get to Armin, the third one.

Inside the walls, people don´t even think about Titans and the Garrison Regiment, one of the military force who “patrols” the city, don´t take their job seriously.
Everything looks very peaceful but it´s nothing but a distorted peace of what´s the world like. The only one who really know the horrible true of the terrifying world they live in, is the Survey Corps. The second military force, the ones who explore beyond the walls and the ones Eren is very excited about.
Just to mention, the third force is the Military Police Regiment, who patrol the city inside the third and most protected city inside the walls.

As I mentioned before, things aren´t as beautiful as they seem. Suddenly, a 60 meters Titan appears and creates a big hole in the wall, letting others titans get through. We went from peace, to chaos, the citizen evacuating the city and much of them dying. Since the first episode we see the reality, titans destroying the city and eating as many humans they can see, and even the force it´s completely in shock. The show shows how terrifying Titans are by the first minutes when one of them grabs Eren´s mother and break her in the middle to scarf her. All that in front of his eyes. That is a very important break down point on our charecter personality, which increase his feeling of knowing the exterior world and generate, consumed by anger, the need for exterminate every single titan. The city evacuated to Wall Rose.

By far, a very chaotic and stressful beginning, but, does the series calms down on its narrative? No. The following episodes we get introduced very interesting new characters, while they train to become soldiers, the tension between some of them and they progress.
Eren, by his speech of how the weak cannot became soldiers, influences most of his partners to enter the Survey Corps, the most dangerous one, after 5 years of training. When just after they had graduated, the Colossal Titan appears again. The situation is the same as before, chaos and panic in all the city and the soldiers. This causes many deaths of characters we got to know a bit, Even Eren seems to die, but, did him?

Suddenly, appears a titan who fight against other titans. Between agony and doubts, Eren Jaeger comes out from that titan. Now we have more questions. Can humans transform into titans?
The series continues by and Eren, using his powers, could took a rock and sail the hole in the wall, with some troubles in within.
The rest of the show is pretty much that, our characters join into excursions with the force, we know new important characters and discover new "smart titans" that are within the humanity.

Characters:

Eren Jaeger: He is not an exceptional protagonist, but it´s great because of that. He entered the Survey Corps because it was his dream and to take revenge for his mother. He is a 15 year old kid who doesn´t really know the world, he doesn´t know if he can even trust himself or their partners. Is a very realistic teen character in a wicked world.

Mikasa Ackerman: I really like Mikasa on this season. She was the best soldier during the military train and we definitely see her show off. She got adopted by Eren´s family, so is very close to Eren, which I personally don´t like about her. In the first season it´s okay, but longer, she is VERY worried about everything that has to do with Eren.

Armin Arlert: A bullied kid who always get help from Mikasa and Eren. He lost his entire family and wants to achieve his grandpa´s goal, go to the outside, discover the world. Armin is an excellent character, he starts being very fearful and nervous on every situation, but as long as the events happen, he takes a step forward and manage the situation without much confidence on himself.

Art and music What can I say, the visuals are just excellent and the music ambient the scenes very well, it has the tone the show need.

This is my very first written review of something, so of course it´s not a good one but I will try to do it better.