Talk to him concerning the Pyramid shirt and OSRS gold following the conversation Visit the Pyramid Program. In case your Agility is finished 51 you shouldn't have a lot of troubles with obstacles. If you've managed to reach the surface of the Pyramid take the item lying there and head back to the NPC which you've previously talked to. Now he'll pay you 10k gold for every Pyramid Top that you bring him.
To start off you want to get into the fantastic Kourend that can be retrieved via boat in Port Sarim. When you get to the port, talk to Veos who is standing south of the inn. Ask him whether he could take you to the Port Piscarilius. From there you should travel west until you find Kourend Castle. In front of the castle gates there's a significant obelisk. Inspect it to get into the cave. You then would like to run south till you see a crack in the wall - squeeze it. Now go west and you are able to fight Twisted Banshees there. There's a safe spot from where you can shoot Banshees that is located in the south of the area.
Though you won't earn as much money here as with other methods, it allows you to also train while earning some excess gold. In addition to that, make sure that you bring some Top Alchemy runes if you can cast that spell because they fall items like Battlestaves and Adamantite Shields.
Prerequisites: Death to the Dorgeshuun, 20 Thieving. You can begin by visiting the trapdoor located north west of Lumbridge Castle. When you are inside cultist cave, then go behind the man which speaks to Buy RS gold cultists and there - just beneath the wall - you will come across a hidden trapdoor. If you do not see it then you have likely not finished the Death of the Dorgeshuun quest. Inside you will find H.A.M. guards and these are your targets.