If you decided to start trading with cryptocurrencies there are a few steps you have to go through.
Step 1 creating a account on a exchange
Create one here make sure to activate '2-Factor Authentication' to make sure nobody gains access to your account with only your password. Now you have to verify your account, by offering them some personal information. This is to prevent money laundry or financial support to terrorist.
Verifying yourself will also allow you to increase your withdrawal limit.
Step 2 Buy Bitcoin or Ethereum
Now that you have created your account, you need to send a Cryptocurrency to your exchange wallet. This because they don't support regular currencies.
Buy your btc here and send the Bitcoins to the wallet address that is shown in the exchange.
(Always make sure the address you send to is indeed a BTC wallet address or a wallet made specifically for the currency you're sending).
Step 3 start trading!
There are 3 markets that represent common base currencies to buy the wide variety of Altcoins that are available on the exchange. If you deposited BTC as your base currency, then got to the Bitcoin Market and pick an Altcoin that you want to buy using Bitcoin.
If you deposited Ethereum on the other hand, be sure to use the Ethereum Market!.