Walk to the Varrock Castle Library and ask Reldo if he's got any quests for you. He will begin talking about the Shield of Arrav and asks you to find a book on one of the shelves. (Make your friend do this quest along with you, it will prove helpful in the end.)
Find the book, read it. it will talk about the Gangs: Black Arm and Phoenix. Return to Reldo, he will tell you to go to Baraek.
Baraek is in the marketplace, he will only agree to tell you about the gang for 20 coins (cheap scum). Pay him and he will tell you about the gang.
Go talk to the Tramp in the alley south of the Sword shop in Varrock. Ask him what's behind the alley. He will tell you about the Black Arm gang.
Choosing your gang:
I chose the Black Arm gang because it was easier, but you can choose whichever one you want, so have your friend choose a different gang from the one you chose. Each gang member has different tasks. These gang affiliations will also be used in future quests.
Black Arm Gang:
If you chose Black Arm then Katrine will tell you to steal some Phoenix crossbows, this is where your friend comes in.
Have your friend go to the Blue Moon Inn and kill Jonny the beard (level 2). Retrieve the Scroll and have him go to the Phoenix Gang headquarters and talk to the gang leader (located in a small house where it will be marked by a dungeon icon on your map).
The Phoenix Gang leader will give your friend a key to the Phoenix Gang's weapon storehouse (located in a house to the east of the Gang dungeon entrance). Have your friend retrieve some Phoenix crossbows from the armory and give them to you. Also make sure your friend grabs the other part of the Shield from the Phoenix Gang dungeon.
Now that you have the crossbows, talk to Katrine and tell her about the crossbows, then enter the upper area of the gang house and search the cupboard to retrieve the Shield half.
Talk to the King and he will tell you to go to the museum to show the Curator. The Curator will give you 2 scrolls.
Take the scrolls back to the King and claim your reward.
Phoenix Gang:
Starting the Phoenix Gang again talk to Baraek he will tell you about the quest. Go to the Gang headquarters and talk to Straven, he'll ask you to kill Jonny the beard and take the Scroll. Go to the Blue Moon Inn and kill Jonny the beard (level 2). He drops a Scroll ("An intelligence report.") Grab it.
Take the Scroll to the Phoenix Gang's headquarters. It is marked by a large dungeon icon on your map in the southern area of Varrock, southwest of Aubury.
Go down the ladder and talk to the gang leader Straven. He will talk and talk about VTM Corporation. Tell him you know who they really are.
He says okay, then ask him if you can join. He says you have to kill an agent and bring his intelligence scroll; you already have that.
Give the Scroll to Straven and he'll let you inside. Get the Shield from the chest in the bedroom, and make your friend get the other part in the Black Arm gang HQ.
P.S. I got this from
www.runehq.com, you can go there to look for quests.