If someone's gotten a cold already this year, will their immunity against the novel coronavirus be stronger?

Not necessarily: When you have a cold, you often develop antibodies to the virus. Those are specific to that cold and not to any other infection, including COVID19. To boost immunity, its best to try antioxidants, vitamin c, or other immune boosting supplements. Additionally, social distancing, self-quarantining at home, and wearing an appropriate mask when going to outside places can keep you from getting COVID1