Necroing an old thread to say please, please, please DO NOT follow the advice from bebeisheretohelp. You don't have to accept any of your results and following on from from what is her/his seeming acceptance at failure, some people will have a C (old style) in Maths for example but need a B or a 6/7 (new style) for a degree they may be taking as a mature student. So sometimes it's not a case of failure, the uni just won't accept anything less than above average and you then need to bump up your own grade to match it. (I do actually have experience with this and my uni application days as a mature student, which is anyone over 21 when they apply)
1- The exams are in the summer assessment period. whatever happens, if you miss the enrolment date, you have to wait until next year for most schools.
2- Decide on what exam board you want to use for your retakes, this will affect what schools you can apply to for sitting them.
3- Sometimes spending money can guarantee a pass, some courses will keep tutoring you after your first attempt if you do not make a grade.
4- If you do not want to use a tutor, then the exam boards will usually have past papers and the answer sheets on their websites which you can download and practice with. There are also online resources like BBC Bitesize which are free.
5- Contact the local schools/colleges around you (dependent on if its GCSE or A-level) at around December time to give your self plenty of time to find somewhere that will take you in time for you to be registered as sitting the exam.
6- Bear in mind that it will cost you to just sit the exam and there is no standard charge. Private may charge a lot more to put off people coming in and messing up their league table results but still make some money from those who can afford it.
7- Ensure you know exactly what grade you need to achieve to prevent wasted time and money. Most are separated into higher and lower tiers so if you need a C or equivalent the lower one would normally be the one to take. Do check requirements especially if they use the old letter grading system and the subject has moved onto the new numerical grading system.