STAY ON TOP OF WORK. DON'T FALL BEHIND. DON'T BE LAZY. DON'T SAY YOU WILL CATCH UP SOME DAY, CATCH UP ASAP!
Seriously though, the amount of times I heard this and said "mhm I'll do it" but didn't end up doing it :/ I can't stress it enough.
Make solid notes from the start so that you can go straight to them during revision.
But the most important thing of all....
Enjoy what you are doing. A levels are going to be tough, no doubt about it. But it will be so much easier if you're passionate about what you do and ultimately enjoy learning.