Bitfinex is a good place to start stocks in cryptocurrencies as is just a trading platform for cryptos.
They have an app for the iPhone but I did not find this useful to actually trade on, but is handy to look at some charts and check trades.
There is also eToro, the social trading platform. Follow traders, copy their trades.
I also like trading on Plus500 as this has other stocks on there too, it’s a general stock trading platform. Their app for iPhone is quite good to use, and can open a quick trade while on the go.
I am sure there are many other good trading platforms, but these are the ones that I trust and are not scammy, be careful if joining other trading sites, and do your research first.