One extremely important way for preparing for exams is through past papers and mark schemes, which are readily available online, through this you are able to learn the style of question you will be asked, how they are marked and start to notice question patterns! Find out which exam board you are with and then visit their websites, the most popular are:

www.aqa.org.uk

www.ocr.org.uk

www.edexcel.org.uk

www.wjec.co.uk

Finally although clichéd your teachers can open the door, but only you can walk through it!

Good Luck! You can only try your best!