If you're talking about how to put an item in your inventory onto your hotbar, then you can right-click one of the hotbars to assign an item to the slot. Use the numpad numbers or left-click the slotted item to use it. You can only assign items that have a “Use” function to them.
There isn't a specific mechanic for game play, but you can do RP actions commands while talking to NPCs, by using asterisks. ex: *pours water from one bottle into another empty bottle.*
