You're new to HTC and you know what S-OFF means? You're farther along than I was when I got this phone.
The latest firmware version is 1.55.605.2 and that may or may not work with Firewater, the free S-OFF exploit. If it doesn't (and there's no telling until you try), you'll have to use SunShine, and that costs $25 (and there's no way around that price). Without S-OFF, best you can do is temp root that goes away when you reboot and may or may not come back (if it doesn't, you do it again). Naturally some stuff doesn't work with that. When you have S-OFF you can flash a custom recovery and then flash SuperSU for permanent root, or better yet you just flash a ROM.
There's about to be an update that will almost certainly break S-OFF, at least until SunShine catches up. I'm guessing Firewater won't be updated.
HTC One M8 / Verizon / CleanROM