Many firms prefer to employ people with some appropriate experience for jobs dealing with people.
Working with a sales support team would give you a particularly useful experience when applying for sales jobs. Once you are in employment, you can gain relevant qualifications through part-time study. Competition for jobs in marketing and advertising often means entrants have higher qualifications. Administration and office-related jobs in a marketing or advertising company can be an entry to this industry with good GCSEs. Working in retail can be a good route into sales work.