To begin with, kill a mob and loot the quest item, and then use the item on somewhere that I can click so that i can finish the quest, and finally turn in quest.

Wrobot is the best bot I have used, Im learning something of it, hope someone can help me. ty :)

Use 2 quests with the same id. On the first step ("Pulse") you get the item and as CompleteCondition you can check if the item is in your bags (code snippets all over the forums).
On the second one, just pick "UseItemOn", add a waypoint and it will use the item there. No need to put any custom complete condition.