Okay to start agility you will need to go into your spell book, select Pking and teleport to the Mage Bank, then walk southwest to get to the agility course.
[You must be registered and logged in to see this image.]Once you are at the course you can do every obstacle except the rope swing, you will need to get 50 agility before you can swing across it. Although the rope swing does give the most experience so you will want to swing across it repeatedly until you get to 99 agility. When you swing across it you will want to immediately click the spot where you were standing so you run over the spikes back to that spot to make it faster to gain experience.
[You must be registered and logged in to see this image.]WARNING!: If you are to slow and happen to click the spikes when you are running over them you will be stuck above the spikes and you will be unable to move. This is bad as you cannot teleport because it is in level 55 wilderness. There are three possible ways to get out. 1.) If an admin is online he can teleport you out. 2.) You can follow someone out of the spikes. 3.) Someone can kill you.
[You must be registered and logged in to see this image.]Thanks to Andrew for the assistance!