Revision when to start and how !!!!!!

Unless you're particularly behind in a certain subject, hardcore revision won't be much help yet. Honestly, I'd advise you just to relax during summer, as you have a difficult year ahead. If you are feeling anxious and really want to start, just try revising content that you've already covered, perhaps focusing on stuff you're not 100% with.
I've just done my GCSEs and from what I remember most people started revision around Easter time, though some did start around 2 months in advance. For some people, 2 weeks before your first exam will be enough but it just depends on how confident you are and what grades you are currently getting in class (in comparison with those you aim to get). I wouldn't advise you to leave it too late though, as you will more than likely find yourself stressed out with too much revision to get done.
That being said, don't stress out if you don't feel ready in the days before exams. For me, revision in the day before each exam was vital and allowed me to learn most of the spec. It depends on the person I suppose, cramming has always been really effective for me and so I had no worry in doing this. Just think back to how you revised for previous exams, or even your mocks, and question if it was effective. If the answer is yes, stick to that method and you'll be fine.
If not however, there are loads of different ways you can try out. For me, I found the easiest way was to read through revision notes, or the revision guide, and then almost "say" the notes in my head from memory. I'd carry on doing this until I was fairly confident, and then try again writing it out. Once I got the bit I had been working on, I'd move on to the next section/ page. I would always revisit the knowledge over and over again and I found this was a really effective way of keeping stuff memorised.
If you're stuck on which method would work best for you, do some research and try them out. You could make notes using your revision guides, make mind maps, write out question and answer flashcards (but remember to use them!), watch revision videos on YouTube, and even listen to podcasts on GCSEpod. Another thing which I found really useful was post-it notes. Particularly for history, if there's anything that you just can't seem to remember, write it on a post-it note and stick it somewhere in your room, perhaps by your bed. You'll find that you'll pick up the information just by looking at it whilst in your bedroom - It's really helpful! Highlighters will also be your best friend. Go pick up some, along with coloured pens, post-it notes, and flashcards; you'll be all set.
Whilst doing revision, I was always told that you should do 30 minute slots with 5 minute breaks inbetween. However, I found that this didn't really work out for me so I'd usually just take a break whenever I got to the end of a page, or was simply too tired to continue. During these breaks, try to do something you enjoy, such as listen to music. But try not to take too long, as a 5 minute break can easily turn into a full day, or even a week, of not doing any revision. I'm aware that some people also liked to use a Revision Timetable in order to organise themselves. These are great if you have the strength to stick to one, but it just wasn't working for me. There are many websites that will help you make one so try and out and see how you feel. One key tip: Turn off your phone whilst you're revising. There's nothing worse than your phone vibrating with messages and notifications whilst you're trying to concentrate. It is just a distraction. You can even give it to your parents if you find it is stopping you from working.

It's likely that you'll find yourself fed up with no motivation whatsoever at some point during this year. I know, it's hard, but just remember that everyone has gone through the same stress at some point in their life. Try to confide in your friends, see how their revision is going along, and you can even find a partner who you can revise with - make sure their not a distraction though. Remember that you have around 3 months of summer after your exams, where you can do absolutely nothing. It's a great feeling when they're all over, believe me. Make yourself proud so you can relax during these 3 months, knowing that whatever the result, you tried your best. The last thing you want is to be haunted with regrets of how you could have tried harder.
And anyway, the fact that you're already thinking ahead just shows that you're going to do great. Good Luck!