I'm trying to fill a few gaps in my understanding about  bitcoin forks will take BCH as example
1) Who determines the value of the new forks when it started {BCH at 2017/8/1 }.
2) if new forks starting from 0 how can claim my BCH {get my free BCH }.
3) after new forks starting it will have huge demand and a few supply why it keeps increasing. 
The market should determine the value. But of course, as you can see from the past two BCH and BTG, some exchanges immediately put them up as futures before the forks are even there, crediting all account holders. Then, it is up to people to place orders, selling or buying. You'll see instant variations in early hours before settling on a price people feel is worthwhile.
Read up on how to claim fork coins, always refer to the official site first, but wait a while before doing so, in case of scams. Make sure it is recognized and stable before even attempting.