Different schools will have different set ups. Most schools have some options which are compulsory, such as English, maths, science, ICT, citizenship, religious education (RE) and physical education (PE), as well as some options left for free choice.
There are three main things you need to think about as youre picking subjects:
1. What do you enjoy?
2. What are you good at?
3. What will help you in the future?
You might be considering subjects you havent studied before, such as psychology or economics. In this case, it is important to do some research beforehand to make sure youre making the right decision. Talk to your teachers about what the course is like and have a look online at the curriculum and teaching materials.
It is possible to take some subjects as a double award, where you attain two GCSEs for a single subject. Science is a notable example of a subject which can be taken in multiple ways. The most common options are Triple Science and Double Science. For Triple Science, you study each of Biology, Chemistry and Physics. Some schools allow you to choose two of these sciences to study individually, while others offer the Double Science award, where you study all 3 sciences but to a lower level, achieving 2 GCSEs.
It is possible to take some GCSEs as a short course, the equivalent of half a GCSE. This is really just helpful if you want to study a subject but dont want to commit to it as one of your main options (it wont carry much importance for any employers/ universities).
Although it may be a bit early to be thinking about what you want to study in the future, it could be useful having an exploratory look at university requirements. Maths and English Language at a minimum of Grade C (= Grade 4) is usually required. Some universities ask for a specific number of GCSEs above a C grade. Top tier universities typically look at the proportion of A* GCSEs.
Some institutions are also very specific about what they look for in a candidate. UCL currently requires a minimum of a C in a foreign language (but is currently the only university to do so). However, they do allow you to take an accelerated foreign language GCSE while at university if you do not already hold one, so not having one doesnt rule UCL out as an option!
Some degree courses may have grade requirements for particular subjects. For example, a grade B in Maths is often needed for Business Studies or Psychology (a grade B in another science is often also required for the latter). Additionally, some universities asked that grades are achieved in one sitting, and do not accept resits.
In 2010, the English Baccalaureate was introduced. While it is not a qualification, it is a measure which recognises achievement in some of the most valued GCSEs: pupils need to achieve a grade C or higher in English Language, Maths, History or Geography, two sciences (Computer Science now counts as a science) and a Modern or Ancient Language. Its likely that your school has made these compulsory, but if not, these are good options for you to take.