In the first chapter of Destiny we have been used to it after the expansion The King of the Taken, to have three subclasses to each main class, but in Destiny 2 it won't be like that. At least not immediately, in fact after finishing the prologue you will find yourself with only one subclass available: The sentry for the titan, the blade of dawn for the sorcerer and the blaster for the hunter.
Le other subclassesi will come unlocked by leveling up: the first around the tenth level and second around the twentieth. The Titan will unlock for example as additional subclasses first the attacker and then the hammer of the sun. But remember are indicative numbers, it could happen sooner or later!
This is due to the fact that just leveling up will not be enough you will also need to drop the relics carrying out a public event, it does not matter if it is normal or heroic. Once this is done will be enough for you continue to attend public events, do missions e playing in the crucible, killing mobs or opponents until the relic will not be charged.
Once this is done you will see a mission on earth called "Fragment of the Traveler", which yes will take place in the European Dead Zone e once completed will give you the chance to use the "new" subclass. For the third subclass it will be the same thing, but the mission "Fragment of the Traveler" will be different than in previous times.
For who had not yet understood like find the relics here is a explanatory video that will help you in the enterprise.
Now you just have to choose yours Harmonization, which is none other than one of the two development options in the subclass skill tree and use your upgrade points to unlock new skills and dominate the solar system.
Se you are worried di failing to unlock enough upgrade points, before the end of the campaign, do not be afraid, since there are plenty of them, thanks to the multitude of activities present in the game.