I suggest CEX (
https://cex.io/). It has a limited number of currency pairs, but their security is great which is essential for dealing with crypto.
As mentioned, CEX is a poor choice. CEX's most recent accounts show that they are in debt and do not have the assets to cover their debt. The assets that they do have are "currently located in a war zone and are not accessible" (direct quote from their most recent financial statement available here:
https://beta.companieshouse.gov.uk/company/08757996/filing-history/MzE4NDczNjEzOGFkaXF6a2N4/document?format=pdf).
They are way, way down the list of exchanges at 78th by volume, with a 24h volume of only $8 million. You would do better sticking to high volume and more reputable exchanges like Binance or Huobi.
I also advise against CEX. I never really liked this service.
The exchange is best selected in terms of the largest possible volume - the list of exchanges can be found on
www.Coinmarketcap.comOf course, fees and service history are also very important.
At the beginning you should start with very small amounts, because cryptocurrency world is a high risk market. To be good in that look for Traders channel here on forum and on Telegram. Read, read and read - it is the key.