If you have one, you could use a character booster to instantly make yourself level 30, but keep in mind that this has some consequences, namely that you won't be able to do the campaign missions for the main game, Curse of Osiris, and Warmind, and that you may miss out on some class-specific items. If you don't want to do that, just play the game. You'll reach level 20 once you beat the main campaign if you haven't already by that time. Your friends could always help you with missions, as well.
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That's not true you can do the stories. It's a good thing to use entirely. The issue I see is if it doesn't give you subclasses upgrade points.