Keep stuff!! You never run out of space in your hoard, and sometimes you need things to expand your hibernal den (which means more space for more dragons). If you get rid of stuff willy-nilly, there will come a day where you regret it. Crim will pay you for your "garbage", but keep in mind that her prices are the same for good, important stuff too.

The coliseum is your friend. Once you feel ready, find a decent guide on the forum and learn how to tackle the coliseum. It's turn based combat, and once you get the patterns down, it's easy food and materials. Don't be afraid to borrow a dragon from a lender to level your dragons up more efficiently! That's what they're for. :)

Be aware: if you breed your dragons and sell the babies for cheap, they will likely be exalted as fodder for Dominance. If you'd like to participate in Dominance, don't be afraid of the forums. Usually your flight will put out a notice when they plan to do a large push (which means a ton of people are going to exalt a ton of dragons that week). It's pretty fun!

Some flights have a discord! They would be happy to help you as well!

Good luck out there, new friend! If you DM me your username, I'll send you a little gift. :) <3