First of all, how old are you? If you're 17 or 18, you need to do some A levels and GCSEs in Maths,English and science
If you're 16:
I guess what you would need to basically self-teach yourself the materials (you probably would know some it of anyway). I would choose 5 to 10 subjects you would do including Maths, English and Science.
GCSE Maths
GCSE English
GCSE Single/Double/Triple Award Science (You have to do at least double for science-related jobs)
Other subjects you could consider:
Languages, Geo, History, Arts (drama,music,art), and many others ...
You can see the many gcses exam boards offer by looking at their websites. Just type: Cambridge/Edexcel/OCR/AQA/WJEC GCSEs into Google and you should see a link that will allow you to see the list of GCSEs they offer. Look at the spec and past papers to see if you know a lot of the material that will be covered by the gcse already.
Find schools in your local area which do GCSEs and see if they're willing to enter you in as a private candidate and for which exam boards and subjects
Then, settle on a list of subjects you would like to pursue and their exam boards
Example:
CIE IGCSE English Language
CIE IGCSE English Literature
Edexcel GCSE Maths
Edexcel IGCSE Biology
Edexcel IGCSE Chemistry
Edexcel IGCSE French ....
Then, buy yourself some textbooks for those subjects along with other resources. You can use a private tutor to clear up any doubts and I recommend you self-teach yourself like it's a school day (from 10 am to 7 pm probably - obviously with breaks)
Hope this helps and don't hesitate to ask questions