You have to be really good with your time management. You tend to be expected to do a lot more work on your own outside of classes, and there's a lot of private study. There's also a higher work load, so you need to plan your time carefully. It's going to be harder and in more depth than GCSEs, so I also think it's important to have a real interest in the subjects you are studying.
I panicked before very exam and stressed ridiculous amounts before my first AS exam, but turned out that once you calm down and sit with a clear head it becomes easier- as long as you do sensible amounts of revision and don't waste all year partying, then they can eb relatively straight forward!!