I just wonder, since im completely new to the stock exchange
This is not stock exchange but this is cryptocurrency market.
If I invest 250 dollars, can I loose them if the price goes down? Or do I still hold my share.
You still have it and just have to hold it until its back to its the same value or more. You only lose if you're going to sell at losses and that's what you have to remember. Whether you trade in centralized exchanges, or with the etfs or decentralized exchanges.
As long as you won't sell and it is projecting you lost value, you still have the chance to recover with that.