I not too long rooted my Samsung Galaxy S2 (GT-I9100), before now I had always been afraid to root my device as most of the tutorials I found online required flashing it with another kernel or using Odin and a computer. This put me off from rooting my phone, until I stumbled on a tutorial on XDA. And by following the easy steps there I have been able to root my device twice. The first time was when it was running Android 4.0.4 (ICS) and then when I had flashed a leaked Android 4.1.2 (Jellybean) ROM.
If you own a Samsung Galaxy S2, you also can root your device by following there steps

  1. Download Root_SuperSU.0.98-Busybox.1.20.2-signed from Dropbox, 4shared
    or XXLSJ- (for Galaxy S2 4.1.2 XXLSJ)
    and copy it to your external memory card (microSD)
  2. Boot your device into recovery mode by Turning off the device and then pressing and holding "The Power button", "The Volume Up"and "The Menu button" all at the same time, and letting go of the buttons once the phone turns on
    If you have done this right you should see a yellow robot, if you dont shutdown the phone and repeat this step
  3. Select apply recovery from external card
    Selecting can be down by using the volume button to move up and down, power button to select/Ok
  4. Select Root_SuperSU.0.98-Busybox.1.20.2-signed and let it install
  5. Once installation is done/finished, you can use the volume up and power button to select .. to get back to the first menu where you can reboot your device
  6. Select Reboot and wait for it to boot normally, you would see the SuperSU app in your app drawer

If you have followed these steps you should have a rooted phone

NB: Rooting would void your warranty, please always make sure you backup your important data