If you just begin in trading, I suggest you to trade with only a small amount. Put the majority of your crypto in a secure wallet and practice tradi g with rest. Like that, you can train, learn about training without the risk to lose all your investments. With time, you will become more confortable and you could trade with higher amount.
There are plenty of trusted exchanges. I suggest you Binance and Bitstamp which I use without any problem but there are many others.
For the rest, it is up to you. Just don't put all your money in one basket.