How to boot from usb on Inspiron 15 558?

Ensure you make installation media with Dell Backup and Recovery in advance in case you want to revert to Windows:

http://dellwindowsreinstallationguide.com/dell-backup-and-recovery-1-8-1-71/

Alternatively download an installation .iso (of the correct Edition):

http://dellwindowsreinstallationguide.com/download-windows-8-1-retail-and-oem-iso/

To boot with Linux your Bootable USB needs FAT32 formatted. Use Rufus to make a Linux Bootable USB using the GPT partition scheme for UEFI computer ensuring its Fat32 formatted:

https://rufus.akeo.ie/