Choosing a subject at GCSE level, depending on the grade you obtain, will give you more opportunity to study it at a higher level, such as AS and A Level, an apprenticeship, and/or find a job in that subject. You will have more choices in the future if you have GCSEs at grade C and above in English, maths and science.
Your school will have compulsory courses, including English, maths and science, which you will have to study as they cover essential knowledge that everyone needs for the future. There will also be a wide range of subjects that you can choose from, and your school will be able to give you a list.
Most learning is in the classroom, including both theory and practical work, and exams are based on a mixture written exams and on-going assessment, depending on the subject.