Hello and welcome to Android Forums
Vuk 
So I want to install CyanogenMod on my Samsung Galaxy S5 G900F and I have some questions.
Would I need to root my phone to do this?
Backup everything on your phone prior to rooting or flashing the CM13 Marshmallow ROM, which requires a Factory Data Reset before installing. See,
Backup! Backup! Backup! NOW!
To root your phone you will need to flash a rooted CF-Auto-Root kernel for your model of phone, via Odin. See,
[SM-G900*] CF-Auto-Root
You will also need to flash a custom recovery, such as TWRP, for your phone. See,
TWRP for Samsung Galaxy S5 Qualcomm
Make a note of your firmware and Product code in case you need to revert to stock. See,
#1.8 of
41 Galaxy S5 models - Dummies Guide
Make a
nandroid backup to your external SD card using TWRP.
Can I get back to stock TouchWiz software in future?
Yes, by downloading and flashing your stock Samsung firmware, mentioned above or by restoring your nandroid backup.
Would this void my warranty?
Yes, as it will trip the
Knox Security flag. Not even by reverting to stock firmware can you un-trip it.
Because this is my first time doing this do you have some other tips for me?
The best place to post for rooting questions is on the
Root - Samsung Galaxy S5 section of the forum.
For links and details of the Marshmallow, CM13, CyanogenMod ROM, see,
Samsung Galaxy S5 gets Marshmallow
These ROMs are released nightly and are
beta test ROMs and require some amount of experience and a lot of reading of the various threads before flashing.
Read, read, read, everything before asking questions, rooting or attempting to flash CM13 and/or gapps.
ironass motto... "
Flash in haste... repent at leisure!"
