how can be a useful guy?

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i am 20 years old this year, i think i am not a child...so i wanna do some meaningful things tnen others can think i am a useful person....the quesion is i don't know how to do it i have made many troubles when i was a little boy........many guy think i am a naughty boy........ so i wanna change it............who can help me??? my english is very poor i am sorry~!