Hey, sorry if it is a stupid question,
But what exactly gives value to the shares? I mean how is it decided in case someone wants to sell his shares.
And the three montly dividends, is it payed in btc or in the form as shares?
It is determine by supply and demand. At first, you will be given shares and by calculating how many btc you put in and the share given, you can find out the cost of each share and that would be usually the starting price of the market. However, demand or supply in the market will change over time so the price of the share will go up or down.