You can apply to a library service for a job as an assistant if you have some experience of working in one, or in an archive or information service, for example as a volunteer. You may need GCSEs grades 9 to 4 (A* to C) or equivalent, including English, to apply.
Experience working in an administration, customer services or IT role would also be useful for applying directly.
For some assistant jobs, for example in industrial or commercial libraries, you may need qualifications at A level standard or higher. You may also need specialist knowledge like languages or science, depending on the job.