1) Work hard from the beginning. I found y13 waaay easier than y12 because i worked from the day we began y13.
2) If you start year 12 & pick a subject and you find you dont like it within a week or 2.. change it asap. Trust me, i stuck to a subject that i didnt like as much anymore (French GCSE didn't prepare me for AS) because i didn't want to let down my teachers.. i got a C in it but definitely could have got an A in something like Politics which i wanted to do.
3) Leading on from point 2, don't listen to your teachers. I didnt pick Politics because i told it would 'not compliment' the other subjects i wanted to do & i completely regretted it throughout the entirety of my AS.
4) Do as many extra curriculars as you can eg taster courses, work shops.. it will help ypu write your personal statement much more easily, & to a higher quality and will help you decide what uni course to do, if you want to go to uni.
5) Enjoy yourself lol as much A Levels are hard, enjoy your college/sixth form years. Make new friends, learn new things.. Sixth form for me was definitely more enjoyable than secondary school.
Good luck xx