Hi and welcome. To answer your questions, first off, rooting and flashing custom ROMs is a little more involved on an HTC compared to most other Androids, in particular phones like Nexus (Google) devices and Samsung's range, all of which a fairly simple to root and flash with custom ROMs.
First of, whatever you plan on doing you need to unlock the bootloader of your HTC One via the HTC Developer website (link here
https://www.htcdev.com/).
Click on the 'Unlock Bootloader' icon and follow the instructions to the letter (when choosing your device choose not listed, most recent phones are not listed).
Once you've unlocked the bootloader you can flash a custom recovery, followed by a custom ROM, such as the Venom. However, the initial steps to flash a custom recovery are all done via the android debug bridge environment (adb), which is a physical connection (USB) between your PC and your HTC One, the latter has to have USB debugging enabled in 'Developer Options'.
I strongly advise to read as much as you can on the subject of rooting and flashing custom ROMs and firmware updates and how the bootloader (hboot) version influences any choice of ROM and more to the point, the firmware. You can find out all you need on this forums 'all things root' section, or on the XDA forum here
HTC One - xda-developers
There is also another important item, the so called S-status, i.e. S-On or S-Off. Out of the box all HTC devices are S-On, which is a security flag in the bootloader that prohibits certain flash attempts that try to change system critical elements of a partition, such as kernel flashes and of course bootloader changes.
In order to make changes to these system elements your phone has to have the S-Off status. However, if you just want to change the recovery and flash a custom ROM this is strictly not necessary, but if you ever make a mistake, say you flashed a ROM which was not intended for your device, i.e. a different CID code, you might not be able to recover your device if it is S-On, whereas if it is S-Off this will be possible.
Another thing that is crucial is to get the correct and properly working USB drivers for your PC environment, which can be quite a challenge at times, but you will find alternatives to get it going; if you have a Windows PC then I'd recommend Kouch's Universal Windows Driver Setup, which was the only package that worked for my Windows 8.1 setup and can be found here
https://github.com/koush/UniversalAdbDriver
To summarise, my recommendation for your proceedings would be as follows:
1. Unlock bootloader
2. Install correct USB drivers and check that your phone is recognised properly by returning your device Serial number in adb
3. Check your device info (adb - type without quotes 'fastboot getvar all'
4. Note down your CID code (in case you want to return to stock), hboot version (for compatible firmware, if you wish to update it)
5. Get your device S-Off, although not strictly necessary, but highly recommended (the only way to remove the 'tampered' flag for example is to be S-Off, but this is not the only benefit)
6. flash custom recovery and root
All of this will keep you busy for a while. As mentioned at the beginning, read as much on the subject as possible and don't pay too much attention to some indifferent YouTube videos.