Exam boards now have a lot of resources to help students and teachers do them. Take AQA GCSE for example...
This is what they have on their GCSE chemistry website under teaching resources.
First, the command words and the subject specific vocabulary are must reads.
The command words are a list of words for the exam questions which explain what the question expects based on the word.
The subject specific vocabulary contains terms which you need to know for your investigations (which make up at least 15% of each paper) and are also a must read.
The maths skills leads to a page explaining the proportion of maths in the exams and has resources for teaching maths and science skills in GCSE. This is crucial as maths is 10% of biology, 20% of chemistry and 30% of physics. However, a lot of maths questions, particularly in paper 2, will be based on maths skills.
Next, there are handbooks on the required practicals which are very useful revision tools. Required practicals make up 15% of the papers, yet it may have been two years since you did some. As well as youtube videos, the handbooks are very helpful reminders of what you need to know for them.
The schemes of work, Teachit and the 6 week teaching pack are not too useful to students, but the sample AO2 questions and mark schemes will give a great idea of the types of questions you will get.
Finally, you can see what textbooks AQA recommends. This is good as the textbooks it recommends will be close to the specification and so you will know that everything you learn in the textbook will be relevant for the exam.
I haven't even covered all of the past papers the exam boards have on their websites, which are always very useful.
So be sure to check your exam board's website for all of the help they can give.