Most jobs at that age are unskilled, so not knowing your GCSE grades shouldn't be an issue. Writing down the predicted grades never hurts, but it's not going to be the sole reason you get a job at this age (or any age for that matter).
Working in hotels and restaurants is always a good start and can be quite good fun. Some won't take on people younger than 18 if they're working near to alcohol, but many places are still happy to. Glass collecting, waiting on and server duties are all fairly standard.
Supermarkets are also good places to have a go at applying to. Again, being younger than 18 and working around alcohol can be an issue but they seem to be well aware of it and have methods in place.
If all else fails, speak to people you know who are older (perhaps siblings, other people in your class or other years, etc) and see where they found jobs and try applying there. Spend a day writing cover letters and sending out job applications and see where you get to. Just look in your area for all the different sorts of businesses there are and apply to them - Either directly or through their website. It's fine to have a sub-par job as your first one. Stick with it for 6-12 months and then find something better. It will be easier then because you'll have some workplace experience.