The Galaxy S5 is only being sold in 16gb variant for now. You could drop a few dollars on a micro sdcard, but if you don't have one you may find yourself running out of memory pretty fast. When this happens you are going to need to know how to clean house. Wiping data is a good way to restore your phone back to factory settings. You will free up space as well as take your phone back to an out of the box state. This is also useful for prepping your device for sale, or loan. This is also a great way to get your phone back to a working state if you mess something up by installing something that you maybe shouldn't have.
To Wipe Data and Factory Reset The Galaxy S5:
Open up settings, head to the "user and backup" section, choose "backup and reset", click the "factory reset" option, read the warning and then scroll down to the bottom and press "Reset device". Enjoy a fresh and clean device.