What error are you getting when you try to install a custom rom?
What ROM are you trying to install?
Have you tried re-downloading that ROM?
Did you make sure to wipe cache, dalvik, data, and system?
Root would normally be installed with the custom ROM, if not you can always install SuperSU after you install the ROM, which TWRP normally offers to install for you, or via a flashable zip (http://download.chainfire.eu/370/SuperSU/)
If you have adb tools installed you can also sideload the ROM. (adb sideload ROM-NAME.zip)
Your best bet would be to follow this guild to return to 100% stock. Hope this helps, just make sure to read through the guild carefully.
thank you verry very thank you