For safely trading it's better that you should join the top 5 exchanges so you can assure that your funds will not be frozen in case you will need it anytime.
https://coinmarketcap.com/exchanges/volume/24-hour/You can follow that link and find the best exchange that you like especially those with smooth interface that you can easily trade.
As newbie on trading the first one should be important is the interface of particular exchanges, such as how they work, are
x exchange is friendly user? easy to use? But as advice to newbies out there, you should also consider the trading volume, just like what BlackFor3st said.
You can use the given link above to also check which exchange has huge trading volume and that will lead you to choose a good exchange.
Since Binance is much popular now and has also a good volume, I can advice to also use this exchange since they have already mobile application while other exchange don't have. Try to browse this page of Binance Academy which has tutorial about trading and also about how to use the Binance Exchange :
Tutorials | Binance Academy