[Verizon HTC One (M8)] S1L3nTShaDoWz:
1) The best most efficient way of backing up your device is to make a Nandroid backup with a recovery such as CWM or TWRP, but of course there will always be times where you could do something simpler to solve your problem such as disabling the Xposed framework but how? My phone is stuck in a bootloop, how would I disable it? The answer is when installing the framework make sure to first in the settings select to create flashable zips (manual) then once they are created go back and do the 'Classic' method. Now I'm sure if you're as new as you say you are you most likely don't know what Xposed is, it is a separate framework which allows you to customize your ROM to do things similar to what a 'Custom' ROM would normally always do. Most people don't think before installing these modules and end up soft-bricking their device and have to use their Nandroids to restore them but the way I mentioned is far simpler.

2) The utmost best method currently available (and only available) for rooting, unlocking, S-OFF & etc is currently Sunshine, you can read up about it here. Sadly and or not so sadly it costs $25 per device but it will do everything you will need to get started such as S-OFF (Security OFF, S-ON prevents writing to the /system partition and some other things I can't remember), Bootloader unlock & root.

3) Personally I prefer TWRP because it just feels like it has more ease of use towards it but thats just my opinion. There is an alternative to it which is CWM as you mentioned or 'PhilZ' CWM, Philz CWM is a touch-based version which to my experiences is a bit buggy but it works. I personally recommend TWRP but you might as well play around with them and see which one you like best.

4) Biggest danger of them all: To my knowledge that is, attempting to revert to S-ON. That is the most dangerous thing I am aware of at the moment but there may be something more dangerous but from what I know that is the most risky thing that is HTC based and or anything based.

This can be considered a good "All in one guide" for you to follow, it has pretty much everything I can think of at the moment.

An alternative to that one would be this one excluding all of the stuff involving Weaksauce and Firewater as Firewater does not work on newer devices and already rooted users don't have a use for Weaksauce. Still though it's a good tutorial for flashing a recovery and rooting.

Here is a solid tutorial on how to revert to stock if you ever decide to (once more do NOT attempt S-ON).

& if you're interested in using ROM's for other carrier variants of your phone then this one may be of interest to you.

Lastly, if you're concerned about any warranty things then as long as you do an RUU like the one mentioned in the 'Reverting to Stock' thread you will have no problems with it even if you're bootloader is unlocked and you're S-OFF. Why you may be wondering?? HTC sometimes ships phones already S-OFF/Bootloader unlocked and also I sort of think they just don't really care... you know as long as you didn't mess the phone up.

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If you have any further questions, I left something unclear or you just need me to elaborate further on something then feel free to ask further questions and I recommend doing your own research on these things before attempting them. Good luck and happy modding!