How to be/act more serious?

k, im going into grade 11 i love rap just had to say that lol, but pple dont take me seriously obv because i just tell jokes and usually ge tin trouble, like im kinda popular, like first day of school as soon as i get into cl everyone will be like sup or w/e, and when they call my name for attendance usually pple laugh cause il do sumthing funny, but iv been gettin into trouble how can i act more like my brother, he shows like no emotion, basically because he had a real real rough childhood and doesnt trust anybody, so he takes pple in fights like 3v1, and never laughs, but i just have to be more serious i get into to much crap