Such jobs are extremely hard to come by, simply because it's illegal for employers to give you work for more than 5 hours on a Saturday or for more than 2 hours on a Sunday. (Further, they can't employ you without obtaining a permit from Sheffield City Council, which involves a lot of checks and form filling). The rules that employers have to abide by are here:
https://www.gov.uk/child-employment/restrictions-on-child-employment
Your only chance of finding employed work is probably to ask around among local market traders (which will probably have become much harder since the old Castle and Sheaf Markets closed down) or to ask hairdressrs if they need anyone for 'sweeping up' duties on a Saturday or newsagents if they need any weekend cover for deliveries.
However 'self-employment' isn't subject to the same rules. If you can set up your own business you can work without restrictions. Young people sometimes find work that way by washing cars, ironing clothes, doing basic gardening tasks, etc. (When I was teaching in Sheffield, one of my 14-year-old pupils was earning nearly as much as I was just by writing columns for two national computer magazines each month. It pays to be adventurous!). So perhaps you need to start thinking about what you're good at and who might pay you to prove it.