How can i lose 140lb in 6 months i know its possible well im 15 years old and weight 250lb?

Eat around 1.500 calories per day.

eat less of and try avoid :

• processed foods such as white pasta, white bread, white rice ect
• chocolate's, sweets and any other sugary foods
• oily foods such as fries, burgers , sausages ect

eat more of

• whole grain foods such as brown bread ,brown pasta and brown rice ect
• fruit and vegetables
• fish, poultry ( chicken , turkey ect) and lean meat
• skimmed milk and low fat yoghurt's.

exercise for 40-60mins a day. --> football , jogging ,swimming ect

your still very young so it should be no problem losing those extra pounds

good luck and keep the exercise going !!