You have to take the path to the left that allows you to jump on, and then into, the well, which leads you to a chest that contains the staff. Spoiler for potential equipment, if you want the hint: The Staff of Wisdom, one of, if not THE most powerful sorcery staff in terms of raw magic stat, can be found in the Royal Army Campsite (bonfire of the same name).
Or you can do what my friend did once, get Intelligence and Attunement to 99 for max spellcasting speed and nothing else, and seeing what you can do with that. If you're wanting to go pure, or near pure spellcasting, I recommend leveling Intelligence and Faith relatively equally to 20, then likely favoring Intelligence until you hit 40/50 (the soft cap).
Personally, I keep my attunement at 30 max usually (enough for six spell slots before any items), unless I want a second cast of a single cast spell like Crystal Magic Weapon without using an item, then I put it at 43.
