How can i lose 3.5 stone (49lbs). i'm 17, 5ft9" and 17 stone (238lbs)?

Keep cycling and walking. Only eat good foods like raw fruits and vegetables, complex carbs, avoid simple carbs, sugars, high fructose corn syrups, and fatty meats. A person should eat between 1500 to 2000 calories a day. If you want to lose some weight, cut back to 1000 calories per day. Also keep a food journal and drink water. Green tea with Fennel will also help. The Green tea helps the body burn fat, and fennel suppresses the appetite. Good luck to you.