If you are using coinbase - Please read this.. (You will definitely like it)

The above screenshot is from GDAX, (same company as Coinbase), then you have to select "LIMIT" and buy or sell as Limit order..

Limit order are completely free,,, no transaction fees... really No FEES

If you dont know the difference b/w LIMIT and Market order.. please see below:-

http://www.investopedia.com/ask/answers/100314/whats-difference-between-market-order-and-limit-order.asp