When at school, it is easy to not think about the future and therefore not take exams seriously. This is especially true if you are not wanting to go to university, college or sixth form as you may feel that your exam results are going to have little impact on your career.
Exam results are not everything but having a good foundation education can help you within any career. Most employers like to see a pass in English and Maths at least, as this not only shows that you can cover the basics in those subjects but also that you are willing to learn and take on new information.
If you have not got the grades you wanted at school then it may be worth retaking them. Trying to restudy and retake exams later in life can be harder and may cost you money as well. Education is vital to anyone who wishes to excel in their career and is not just for those wishing to progress in to higher education.