A brave teen fighting a rare form of cancer tells TMZ Sports Carmelo Anthony's gift to his family changed his life ... and gave him a new outlook forever.

We talked to Jarrell Lara ... a 17-year-old Knicks fan battling Langerhans Cell Histiocytosis ... a type of cancer that attacks the bone marrow.

'Melo suprised Jarrell and his whole family earlier this month, presenting them with a brand new car they desperately needed to get back and forth during some financially tough times.

Lara told us the gesture meant so much more to him than Anthony can even imagine ... and that now he has a goal to shoot for in life ... besides beating the disease.

"It just makes me think how if you really try, you could be that successful."