In the Shard Shop, you can buy champions, weapons, or skins without having to spend a single cent of real-world money!
To access the Shard Shop, first tap the shield icon at the upper right corner of the main menu.
Then tap the "Shard Shop" button at the upper right corner of the screen.
The Shard Shop contains a selection of champions, skins, and weapons. The items in the Shard Shop rotate every week, so not everything will be available all the time. If you don't see something you want in the Shard Shop, just wait for a while and it's bound to show up sooner or later.
To buy items in the Shard Shop, you need Champion Shards, Skin Shards, or Weapon Shards. You can earn these Shards by completing daily quests. Once you have enough Shards to buy the item you want, simply tap the item and it's yours to claim!
For more information about how to earn Shards, read this article.