My dd brought an amazing piece of work home the other day, all done on a sewing machine, she had never touched one before. So thinking along the lines of items she will need to encourage her? Machine etc, fabric scraps etc.
Cottonpatch sell lovely fabrics, either by the metre, or packs of pre-cut pieces.
She will need thread too, I bought a job lot of 100 reels from ebay, it didn't cost that much (cheaper than buying individual reels) and it means that whatever fabric I'm using, I can usually find a thread that matches pretty well.
A decent pair of fabric scissors will be a good investment, as long as they don't get stolen and used to cut the tops off coke cans (yes DP I'M looking at you).
Maybe some books with projects in? There are lots on Amazon dedicated to small projects, and sewing with small amounts of fabric.
sewing boxes can be pricey, and tool boxes can be a better shape, so depends on her style (sewing boxes very cath kidson)
cutting out scissors small thread cutting scissors spools/bobbins for the machine coloured threads box of mixed buttons some ribbons/decorative bits velcro elastic pins (make sure they are long ones needles fabric, small bits and some larger pieces
and of course a machine. John Lewis do a very nice starter machine. Costs around £70 new. We got one on ebay for much less. Comes in funky colours
Find a beginners machine project book (bags, cushions etc)
My daughter loves sewing. This is what she got last christmas. She is very proud of things she sews, and it is very useful for present making.
I've got the john lewis starter machine. See if she actually uses it frequently before moving onto a bigger machine. it's perfect for most things that don't use thick fabric (denim/cord etc) or have many layers (a lined blazer would be a bad idea).
I have used mine for quilting, apron making, hemming, stocking making and will be using it for making some pajamas and an advent calendar this month.
And tell her to visit craftster.org too...loads of younger girls on there...there are loads of cool tutorials that appeal to teens and advice from older crafty people. They have craft challenges monthly with prizes...and it's a lovely community.