There are a few sites where you can trade cryptocurrency. Here i have selected some site in terms of flexibility and their reputation.
Coinbase
I will start with coinbase in the top of my list. Coinbase is a trusted and stable site for trading. Currently the have only three coin Bitcoin, Litecoin and Ethereum. For a beginner this would be a great choice. When you invest 100$ for the first time, they will give you 10$ bonus.Changelly
Changely is one of the best exchange palace to buy crypto with your credit card. Besides Bitcoin, Litecoin and Ethereum they also offer Ethreum Classic, Dogecoin, Ardor and so many coin. Their interface is easy to use and all of the function will be completed very quickly.Bittrex
Bittrex is very popular to steemain and trustworthy. Liquidity in the exchange makes trading experience more stable. This is place I first go for trading.Bitfinex
Specialty of this site is more transaction volume, low fee and advanced indicator.My advice will be trade only the amount that you can afford to loose. Also be careful to use e-wallet. It is better to keep money in paper wallet or hardware.
