In Destiny: 1, there are a total of 3 sub-classes for each character Hunter (Gunslinger, Nightstalker, Bladedancer), Warlock (Stormcaller, Voidwalker, Sunsinger), Titan (Striker, Sunbreaker, Defender). But as we know, Ghaul has taken over The Last City and The Traveler. So, when you start off in Destiny 2, you’ll only get 1 of your three sub-classes.
To get back your remaining sub-classes, you need to be above Level 9 to 10.
After reaching that level, all you have to do is complete Public Events. In Destiny 2, after you complete a Public Event, you’ll get a Loot chest.
Open that Loot Chest and you will get you Subclass Relic. It’s a blue/rare item.
Note: Keep in mind, sub-class relic drops are rare and won’t be dropping frequently. So, you need keep doing Public Events until your sub-class’ relic drops.
After that, you need to Charge the relic. It says:
Rekindle you lost abilities by defeating enemies and fighting alongside fellow Guardians in Public Events.
Basically, what this means is that you have to farm Public Events until your relic is fully charged. Just complete as many Public Events as you can for the relic charge. It would take like 5-7 Public Events to fully charge your sub-class relic.
Once its fully charged, it’ll unlock a unique Subclass Quest called Shard of the Traveler in EDZ (European Dead Zone).
Note: Recommended Power Level for this mission is 50. So, if you’re Power Level is below 50, we recommend you to get your level up to 100, so that you can easily get through this level.
Just load the mission. complete it and you’ll get your second Subclass back.