First, disable your Compatibility Support Module (CSM), aka "legacy mode support." Having it enabled complicates the boot path and sometimes directs the boot mode down that road even when you don't want it to do that. While you're in the firmware setup utility, you may want to re-enable Secure Boot. Although it does sometimes cause problems, it usually doesn't. (Many reports of Secure Boot problems actually have other causes.) You can always re-disable Secure Boot if you run out of other options. Secure Boot, as the name implies, is a security feature, so you probably shouldn't be disabling it unless you're certain you need to do so.
If that's not sufficient, use your firmware's boot manager. This tool is usually accessed by hitting Esc, Enter, or a function key (usually F8 or above). Most modern EFIs show two options for booting external boot media, one of which includes the string "UEFI" and the other of which doesn't. Pick the option that includes the string "UEFI" to boot in that mode.
If that doesn't work, then chances are your boot medium was prepared incorrectly. You didn't say what tool you used to create your USB drive, but quite a few are available. Rufus seems to have the best reputation among Windows programs and Unetbootin usually works and is cross-platform. Neither is guaranteed to work, though. Note that Rufus has at least three options for different partition table and boot loader combinations, so be sure to use an option for creating an EFI-compatible medium.
