You need to find an exam centre (=any secondary school or college plus some private concerns) willing to accept you as a private candidate. Any exam centre can but in practice very few will because they're busy enough without the extra work. The entry deadline for this Summer is 21 February at the exam board (after which entries can be made but the prices doubles on 22 feb and gets worse after that) but the centre will set an earlier deadline to candidates as they need to make sure they've done and checked everything before they submit by 21 Feb.
If you've ever taken GCSEs or A levels in the UK, the centre will want to know your UCI (unique candidate identifier) which can be found on most GCSE/A level exam certificates. It's a 12 digit, single letter code of the form 11111 006 2222B (first 5 digits are the centre no. that gave you the UCI, next 3 indicate the year (2006 in this case) they gave it to you, the next 4 are your candidate number from the school that gave you a UCI and the letter is a data check. If you tell me what exam board certificates you have I might be able to tell you where to look.
What subjects are you doing and which which exam board(s)?