Well, it’s pretty simple. Check out this list of Zcash markets:
Figure out what you want to trade for Zcash (USD, BTC, etc.) and then register on the relevant exchange(s) which provide that market. You’ll then just have to deposit your purchase currency to the exchange, use it to buy Zcash, and then (ideally) withdraw the Zcash to your personal Zcash wallet for safer storage.