If you're looking to hand off a Samsung Galaxy S5 to someone else and there's already stuff on there, it's wise to do a factory reset to wipe all of the existing data from the device. This is a pretty straightforward process, but a vital one if you want to keep your personal data secure. Make sure to keep your Samsung Galaxy S5 plugged in and charging during this process, since having your phone die in the middle of it can lead to permanent damage.
How to wipe all data from the Samsung Galaxy S5
Swipe down from the top of the screen and tap the gear icon in the top-right. Swipe down to the User and Backup section, or tap the magnifying glass to type in a search in the top-right and tap Backup and reset.
The last option on this screen is Factory data reset. Tap this, and you'll be given one more warning screen before making the plunge. Review all of the data that will be deleted, then scroll down to the bottom and tap Reset device.
Be sure to pop out the microSD memory card from the device if you're getting rid of it, since the information on there isn't touched by a security wipe, and odds are you've got valuable pictures and music on there.
We have a ton more Samsung Galaxy S5 guides for anybody look for help on other issues.