If you are new to crypto trading, and looking for a platform to trade, here are some platforms I use.
Share with all of you

1.
Binance (Strongly recommended)
2.
Bitfinex (Stable, not many coins available)
3.
Bithumb (
ps:security issue happened, consider well)
4.
Upbit (Korea exchanges)
Welcome to other suggestion

*******Additional information************
Recommended by FlamingFingers :
Bittrex - 2 years exp. and No issue ever
I really do not know much about the last two you mentioned, but for bitfinex and binance, these are good exchanges that have been able to do well over time. I strongly recommend binance as well based on liquidity, trading fees, and then the support which is pretty good for me.
The UI too is okay but since I am always using APIs to trade on tabtrader, I usually do not pay so much attention on the exchange UI. For bittrex, based on loyalty, and reputation, this is one exchange I will never stop using, and even though sometimes, the fees in binance has spoiled me, I still feel confident with the bittrex exchange.