I would recommend reading basics of trading first.
Exchange platforms I am using without a problem and can recommend are:
1.Binance.com
2.Bitfinex.com
3.Poloniex.com
3.Kucoin.com
If you want to buy Crypro with Fiat I suggest:
1.Cex.io
2.Kraken
For tracking prices, the best site is coinmarketcap.com
Good mobile apps for portfolio management: Delta and blockfolio.
Are these mobile apps for iOS and Android devices?
On the list, only Binance and Bitfinex has their own mobile app. Though these exchanges have responsive layouts anyway so using them on mobile phones shouldn't be a problem.