How to install ClockWorkMod recovery on a Samsung Galaxy S2 plus gt-i9105? I have a Samsung Galaxy S2 plus (GT-i9105), running Android 4.1.2 (Jellybean).

I really tried evereything – with Odin, with apps, with other programs. I tried to update with Samsung Kies, but that doesn't work.

I want to install CyanogenMod. I tried with the CyanogenMod Installer, but doesn't work. Now I'm out of ideas. Could you give me any pointers on how to succeed?
First: Cyanogenmod is a customized version of Android and NOT a recovery.

Ok, let's move. 
You should root your device first. Once rooted you can install the ROM Manager App from the Play Store, which will guide you to install Custom Recovery.
In order to root your S2 follow this instructions: http://forum.xda-developers.com/showthread.php?t=2663834

You also can follow this tutorial: http://youtu.be/xBJyQrbEkuc