Snotlout: "The best dragon trainers, like myself, can train multiple dragons. Hookfang totally listens to me, and so do a whole bunch of other dragons. You need to do the same! Talk to Gobber in the Hatchery for more information."
1- Talk to Gobber in the Hatchery
Gobber: "Oh no, lad/lassie, you think you're ready for more than one dragon? Interesting!
You'll find more eggs in Johann's store or complete more quests. Visit the stables and I'll show you what you'll need to do!"
2- Visit the Stables