u'll be fine. don't sweat it girl. really, it'll be ok.
ur first appointment, is an assessment. where they determine where u stand, emotionally, physically, and what u need from them. be it moral support, someone to confide in, etc.
since ur 15, u'll more then likely be accompanied by an adult. which is really kinda sucky, and makes the whole trust issue between doctor and patient that much harder..cuz everything u tell them, (i.e. concerning hurting urself) they by legal law have to tell ur folks.
ur first visit, u sign a whole lotta papers. possibly a 'no harm' contract. u might get weighed, if they wanna track ur progress, in the coming weeks, ( or in a lot of cases.. see if T helps you, or makes u worse.) and u talk about urself. why ur there, what u do, what makes u do it, and tell them how u want them to help. they may try to get other people involved in treating u. group therapy, pychologist ( someone who can precribe meds.. the one ur seeing, can't.) treatment, or just one on one counceling.
its really not that scarey. u'll get used to it. this may be just what you need. it'll suck til u get comfortable.. so give it time, in order for u to get better, this is a step towards happiness. but just keep in mind..happiness doesn't always come right away. so don't get dissapointed if u leave, and ur still a mess..emotionally
this could very well be blessing in disguise, i'm glad u get the oppurtunity to get treatment at ur age now.
and don't even get me started on lecturing about diet pills, and cutting. that stuff will screw u up. physically & emotionally. its no way to solve ur problems.. its a way to die.. but i'm sure u know this. so i won't get started. but if u ever need a ear, i'm just a PM away.
good luck, tommorrow. don't be afraid to open up. they arent out there to get you..they are there to help you.