I think it is okay for someone in high school to have an idea of what he/she wants to join. I did. I grew up around greeks so I had a pretty good idea, but I still went into it with an open mind. Here was my case: I went through the high school group of one sorority, got to school and was interested in another sorority, then on closer inspection I changed my mind and joined SGR.
It would be hard not to have already decided if you have family members (mom, dad, sis, brother, cousin, etc..) who are strong in their organization.
I do agree with what Strive said. I'd like to add that if the pan-hellinic chapters (NPHC, NPC, NIC, IFC) at that school do a good job in fairly introducing all greeks to incomming freshmen, then he/she has a better chance of finding the right organization for himself/herself.
Not to get too far off topic, but, my concern is for high school students who are part of greek-like high school groups comming into college thinking it will be the same. There is more respondsibility at the college level than high school level.