You may be able to start with the Malissa
pattern from BurdaStyle. It's fitted only at the bust, and has a graceful flow to it in drapey fabrics. Depending on how much sewing experience you have, adjusting this dress pattern for you and wrangling fancy fabrics could either be challenging but fun, or something you really need assistance with.
Were I you, I would find a mentor who is willing to help you get started on your way, who can also give coherent advice over the phone for when you get stuck. If there is a home economics teacher at your school, I definitely recommend asking them if they're willing to be your beacon in the darkness.
Stylewise, with Malissa, I think you could do it with a satin underlayer and a chiffon or georgette overlayer, with the sleeve being just the overlayer fabric. That would be super-chic in steel grey, me thinks! Changing the sleeve into a big circular sleeve won't be too hard. It will be somewhat like a bell sleeve
, but with a larger radius. You would need to slash
the sleeve pattern vertically in several places and then spread
the pieces out in a circular manner. You just tape the slashed bits to a giant piece of paper and use a string, pin and pencil to redraw the bottom hem. You'll also need to read up on sewing with fancy fabrics like satin and chiffon, including seam finishes. Your local library may have some sewing books that could be helpful. Google can also lead you to good resources just by searching things like "how to sew satin" or "how to hem chiffon."
If Malissa isn't to your liking, schedule yourself a couple hours to go sit in the local fabric store and just go through all the pattern books they have. Check the sections for regular dresses, too, not just formalwear, because a day dress done in a formal fabric is sometimes just the ticket
I say "a few hours" because that way you'll have plenty of time to bug the ladies and gents who work there.
Best of luck!