It depends a bit on your subjects, but
- 3 black pens
- 3 highlighters
- calculator
- pencil
- rubber
- pencil sharpener
- ruler
- mini hole punch maybe (and if so, those little hole reinforcers so that your pages don't get ripped out of your folder
- water
- planner
- either a day folder or a mini topic folder for each of your subjects - carrying 3-6 big folders around is fine at the beginning of the year, but awful at the end! I started with a day folder but ended up with a mini folder for each subject where I kept the work for the topic we were studying at the time, and any resources I'd need on a continual basis such as exemplar essays, grade recorders etc.
Have fun stationery shopping! Sixth form isn't easy, but it was my favourite part of my whole school career, Just keep on top of things as much as you can, and you'll get so much out of it! Also, I mentioned a paper planner... life is so much easier if you have one that you put all your deadlines and homework and exams etc in. It is something extra to carry, but for me it was always worth it!