To start agility you must first teleport the the agility arena by clicking the teleport interface > Skilling > Agility.
Once at the agility arena follow the route in the below pictures.
1st Log Cross
2nd Net climb
3rd Tree Climb up
4th Tight Rope
5th Tree Climb Down
6th Net Climb
Finally 7th Tunnel Crawl
Once you have completed your lap you will recieve an agility ticket which you can use to claim xp or save to use in the agility ticket shop by talking to the Agility Boss.