i just went to my first funeral yesterday, for one of my old best friends. he's 18.
i wore black slacks with a black-ish buttoned shirt. it was kind of patterny. looking around i saw a lot of jeans and some Hawaiian shirts, even though it was mostly a young crowd. some people just wore regular clothes. i saw some blue jeans with nice black shirts. id suggest some black slacks and a nice button up shirt, w/e style.
EDIT: mostly short-sleeve too, and no ties.
i wasnt expecting such casual attire for a funeral. i thought i came under-dressed, but i guess not