Inventory screen, showing the first page with open slots, and locked slots at the bottom
Your inventory can be accessed from the Main Menu. Tap "Inventory" at the far left to get there.
If this is your first time in the Inventory screen, you will see that you have 15 open slots in your inventory available to you. You may notice there are several pages of your inventory which you can view by scrolling left and right. See a few paragraphs below to learn how to unlock it.
If you've completed a few battles or maybe your first hunting grounds, by now you should have items that you've looted showing up in your inventory.
You can sell or enhance your items in the inventory. Let's look at each option.
Selling: Simply tap on the item you want to sell, and tap the "Sell" button.
Enhancing an item: You'll need to first have enhancing stones in your inventory, then tap on the item in your inventory that you want to enhance. To learn more about enhancing an item, see Item Enhancing.
Unlocking inventory slots: You are given 15 slots to start with. Currently the other slots are "locked", but they can be unlocked 5 cells at a time by using gems to purchase the Inventory Slot Expansion item at the store, or through event rewards. Alternatively you can just click on any locked slot, and you will be given the option to spend 5 gems to unlock 5 slots.
During your battles in the hunting grounds, you will attain many items. Remember to periodically return to your inventory to sell and equip these items, thereby keeping space available in order to loot new items. Otherwise you'll be unable to collect more items until you make room again.