The lighthouse is located far west of Rellekka over a broken bridge. To enter the basement you must have completed the Horror from the Deep quest at least up until you have first entered the lighthouse. During this quest you will have to fight the Dagannoth Mother (level 100) but after the quest you will find that there are many great monsters to train on in this dungeon. For more information on the Dagannoth Mother, please check out the Horror from the Deep quest guide.
The lower leveled Dagannoth only use a ranged attack while the higher leveled ones use both melee and ranged.