Based on the age range and context this term is put in, it can mean a number of different things
1) Middle school age- kissing or making out at most (unless for those few mature sluts who make make it farther)
2) High School- Generally having sex. For most freshman and some sophomores, this means kissing or making out, unless they make it farther which can be common
3) Young adults- sex.
4) Older adults, like parents- Being really nicely helped out
1)John and Lilly are hooking up at the school dance tonight!
2) I hooked up with the quarterback, his d is tiny
3) I think I might've hooked up with Jerry when I was drunk.....
4) Wow!! Bobby really is hooking me up with those concert tickets!