How he died and his wishes. 

Sodapop: He died during in the Vietnam war from tuberculosis in an pow camp. He also had severe mental disorders which developed during this time. After his death he was to be scattered among other fallen soldiers. (age 22)

Two-bit: He died from alcohol poisoning in his mothers home. After his death he was buried by his mother and sister. (age 29)

Darry: He suffered a severe heart attack. Although healthy and fit, an unknown genetic and rare disorder caused his death. After his death he was buried next to his parents. (age 67)

Ponyboy: He became suicidal after the death of his two closest friends and Soda. After his death he was to be donated to science. (age 19)

Steve: He died from a fatal car accident which killed him and two others. After his death he was buried. (age 34)

And we know how they died...

Dallas: (age 18 or 17?) <3

Johnny: (age 16) <3