There are plenty of trading platforms and you can start with the easiest to use one. I think they are Coinbase and CEX. Both are reliable and simple in use, plus, they have a good reputation. On CEX you can easily buy Bitcoin or major altcoins with EUR (https://cex.io/eth-eur), USD, and other currencies. The same with Coinbase. The principal difference between these two exchanges is support of the countries. Coinbase covers 33 of them, while CEX can be accessed from more than 200 countries.