There are plenty of good and trusted cryptocurrency exchange and it up to you what exchange do you want to use. I highly recommend to use the binance exchange and bittrex because they have good security and both of then has good reputation. Binance exchange already have mobile application so just download it in your playstore ot appstore. Lastly, if you going to use a exchange make it is belong to top major exchange and do not try to use unknown or new exchange.