I need help finding a hairstyles for Prom

A half-up style might look nice?  What shape is your face?  Do you have a nice neck and collarbone?  Which parts do you want to accentuate and which do you want to distract from?

It sounds like your dress and tiara will look great!

Okay, I will have a think.

Have you experimented at home with the tiara yet?

Formal dressing is difficult, because most of us don't do it very often and most of the stuff is quite expensive but also rare, (e.g. magazines aren't chock-full of formal clothes, hair and make-up these days).  But it doesn't mean that there's no inspiration available anywhere.  I would suggest that you do an internet search for photos of the Oscar ceremonies and look at what the actresses have done with their hair on those occasions...do any of those looks appeal to you? 

It sounds as though you are quite apprehensive about all of this.  I think you need to get used to wearing your hair up and in slightly more fancy styles.  All you need is some hairpins and day-old hair (i.e. not just washed - freshly washed hair usually falls out quite quickly and it can also be more prone to flyaways).  Try out some different looks at home and when you go out (you might need to practice a little bit at home!)

You could start with this one:

Part your hair a little bit left or right of centre.  On the side that has the lesser bit of parting (i.e. if you part your hair right of centre, the bit on the right of your parting will be smaller than the bit on the left), section off the front bit of hair that runs from an inch or two back from the hairline down to your ear.  This should be about one-fifth or one-sixth of all of your hair.  Section this out of the way, then brush the rest of your hair back into a ponytail about half the way up the back of your head.  Secure the ponytail very securely with a plain band.  Then take out the front section and wrap it around the hairband that secures your ponytail, covering up the hairband and making it look as though your ponytail is just tied up with your own hair.  As you wrap it around, use old-fashioned hairpins to keep it in place.

You can amend this look by curling the ends of the ponytail, leaving out wisps of hair to frame your face (and curling them) or changing the height of the ponytail.

This is quite a simple look so you should try it out a couple of times before moving on to more complicated and formal prom looks.

Good luck!