Hi zygmuntzyzio,
In the client, click "My Stars" then "Rewards Store"
In there you'll find a wide selection of things that you can do with your StarsCoin.
Everything is priced such that each coin is worth $0.01 -- some of the merchandise is rather high priced -- for example a coffee cup is 900 coins.
At the moment, the best us of StarsCoin is playing "the Deal" -- if you play once every twelve hours you will get a return on your money many times more than 1 cent per coin.
Since I started keeping track, the average return is 6.4 cents per coin -- lately it has been more.
See https://forums.pokerstrategy.com/forum/thread.php?threadid=436820&goto=firstnew
Generally I play when the jackpot passes $100K, since the longer you play before someone wins the less you get per coin.
Oh, and you just might hit the jackpot.
Cheers,
VS