If you press the N button, The Store window will pop up. In it you can spend your currency (Credits and Cubits)
The list of items available for purchase is large enough:
Many flasks, random mounts, costumes, most useful (maybe!) classes, some starter bundles, items, many blocks and other.
Some items are available for purchase with Cubits and the rarest and premium ones can only be bought for Credits.
Waiting for a good discount offers is a great strategy when using in-game store.