Some of my friends’ first jobs were:
~Retail- Primark, M&S
~Food- Pizza Hut( I know a lot of people that started there as soon as they turned 16) and other food chains, Local cafés etc
~You can also do a paper route- my cousin did this during year 10 for a bit. Wake up early and hand out newspapers in your area before school.
Although I’m not sure if you’ve just finished year 11 or just starting it, but if you’re just starting it this sept I would advise to wait until the summer after GCSEs (that’s when a lot of people I knew got their first proper job- retail or food chain based- and they carried on throughout college). It might be a bit stressful to balance yr11 school work and a part time job.