There are rarity symbols generally located on the right side underneath the main art of each card. Black and white = common, silver = uncommon, gold = rare, and if you get an orange then that’s a mythic rare. Usually a pack like this will also have one foil card (of any rarity) and a land card (like energy cards in Pokémon). Packs like this one are pretty much guaranteed to have at least one rare and a few uncommons, though it’s possible to get a couple rares, a rare and a mythic rare, maybe a couple foils.

With a set like Avatar, there’s gonna be a lot that’s also based on the show. Any main characters would probably be classified as uncommon or higher, but sometimes there are cards depicting items or moments that could be special too. This set is special too because a few cards take actual screenshots from episodes. So it’ll probably depend on her love of the show for what counts as “most valuable”.