You can access your inventory on by tapping the blue backpack on top of your screen. The number next to it is the amount of available slots you have to store items.
The basic inventory can become full rather quickly. By upgrading it you can stock more items and therefore earn more Coins selling Items.
To upgrade it, open the inventory by tapping the backpack icon at the top on the screen.
A list of items will be shown in the bottom left corner that you need to obtain via animations, crafting, or other ways. Alternatively, you can use gems to purchase the required items.
If your inventory is already full, you can tap the X button on less desired items to remove some in order to obtain the required items to upgrade the inventory.
Remember to CHECK THE MARKETPLACE before deleting items from your inventory! There's probably something you can sell, instead.