Not stressed over my GCSEs at such an early stage. A bit of revision to stay on top of what you've learnt is enough for year 10-11. So many of my friends started to 'get serious' with revision before Year 11 started and gave up when it really mattered, so my main advice is to enjoy the summer and try not to worry too much. If you want to revise, I'd suggest reading guides and making notes. I don't think past papers will be necessary at this stage unless you're really struggling with a certain subject.