Cryptocurrencies can be traded for one another on crypto exchanges in much the same way that fiat currencies can be traded for one another on the FOREX. Exchanges allow you to set up an account with them and deposit and withdraw coins to and from a wallet associated with your account. The benefit of exchanges is that you can trade cryptocurrencies almost instantaneously in a market with high liquidity. The downside is that you are entrusting your coins to a centralized party. Even though most exchanges keep coins in cold storage until you withdraw, being a centralized body they are a big target for malicious attacks.
There are now a large number of cryptocurrency exchanges to choose from but be sure to research an exchange before choosing to send your coins to them. Look for one with a solid reputation and good trading volume as you want both security and liquidity. Always research thoroughly in crypto: you are your own banker and responsible for the safety of your own money.