I want to install CyanogenMod on my Samsumg Galaxy S II. If I install ClockworkMod I think I will void the warranty provided by Samsung.
Must I install ClockworkMod or can I just install CyanogenMod?
Two things are important here:
1.) Even if you wouldn't install ClockworkMod, you'd lose your warranty since you replaced the pre-loaded operating system with unsupported one.
2.) ClockworkMod (or any other alternative recovery) is what actually installs CyanogenMod on your phone so it is required.
In short: Replacing your phone OS with Cyanogen will void your warranty in any case. You'll need some way of installing CM and Clockwork is probably the easiest around.