after GCSE job

Hi. It seems by your post that your GCSEs did not go well?
If this is true, enrol at night school at a local college to relearn and resit your Maths and English GCSEs, as employers will expect you to have achieved a Grade C at least in both and are a basic requirement of all jobs (I don't know what grade number this is equivalent to for the new GCSEs).
These courses will be free if you achieved a D or below. If you have GCSEs in other subjects, state them on your CV. You do not need to resit other subjects unless you want to.

Are you good at things that are more vocational than academic? Maybe you were better at making things at school compared to English? If you have examples of your work, employers will take this into account, possibly over academic achievements if it is relevant to the job you are applying for.

You want to build up skills such as teamwork, timekeeping and reliability. If you have never had a job before you would want to give examples of what you did at school, for example teamwork was required in science to conduct experiments.

Good websites for you could be:
https://www.indeed.co.uk/
monster.co.uk
https://nationalcareersservice.direct.gov.uk/
Also look around your local community for any opportunities and give out speculative CVs and covering letters- not all job vacancies are advertised. Always tailor your CV to the company you want to work for. Employers do not like generic CVs and will ignore them.

If you are more vocational, maybe an apprenticeship would suit you? Engineering and construction apprenticeships are always in high demand.
https://www.findapprenticeship.service.gov.uk/apprenticeshipsearch

Volunteering could be suitable if you cannot find a job or want to gain skills whilst in college or sixth form. Have a look at https://do-it.org/ for opportunities in your local area, it's not all working in a charity shop! Volunteering is valued greatly by employers and will give you skills you can list on your CV when applying for paid jobs.

Here are some useful websites and documents. I know your not at university but they have some good advise that you may find useful:
http://www.port.ac.uk/careersandrecruitment/careersadvice/cvsandapplications/cvs/
Good luck!