The following is a walkthrough for Pier 69 in GTA San Andreas.
Now that you have killed Jizzy B, you have to take out the other two heads of the Loco Syndicate, Mike Toreno and T-Bone Mendez. In addition, T-Bone and Toreno will be attending a meeting with the Ballas in Pier 69 and your former Grove Street Families homie-turned-enemy Ryder will be there, so you can take revenge on him too.
When CJ arrives at Pier 69, Cesar Vialpando will be waiting on the roof of a building across the pier. When arriving, Cesar will hand CJ a sniper rifle. Shoot down all of Mendez's guards to allow the Mountain Cloud Boys to take their place. Eventually, T-Bone Mendez and Ryder will arrive with their escorts to the deal, but, in an unexpected turn, Mike Toreno arrives in his black Maverick helicopter and sees the guards' bodies on the roofs, so he flees. Toreno's flight alerts Mendez and Ryder and a huge gun battle erupts between the Ballas, the San Fierro Rifa and the Mountain Cloud Boys.
You will then enter and fight Mendez's goons while defending both Cesar and the Mountain Cloud Boys. Don't jump into the battle right away; continue using the sniper rifle from the roof to pick off any enemy gang members you can see in the street (some will begin running towards your building and will come upstairs and attack if not stopped). When you do decide to head to Pier 69, make sure Cesar follows you (so take the stairs down; don't jump off the front of the building). You do not need to kill everyone to reach T-Bone, and you don't need to run through the middle of Pier 69 which becomes a shooting gallery; simply run along the east side of the pier, by the water, picking off any enemy that jump out at your or shoot at you from between buildings. When you reach the end of the pier where T-Bone is, you can shoot him down, which triggers a cutscene showing both CJ and Cesar shooting T-Bone, whose corpse then falls into the water.
Ryder then tries to make a getaway, he jumps off the Pier and starts swimming towards a Speeder boat, you must kill him by either swimming towards the boats and giving chase doing a drive-by or use a gun and kill him from the pier, either way works, though if you use the boat drive-by option you'll get a phone call from Cesar checking up on you. The mission is passed but Toreno is still on the run.