Hold down the Power and Home Button for 20 seconds.
I have tried it but it doesnt go into DFU mode
Try following this tutorial: http://www.iclarified.com/entry/index.php?enid=1034
Out there if you can't figure it out I highly suggest you don't JB and as posted iClarified is one of the better ones if not the best. not trying to be mean but there is way too much info on JB there isn't much anybody here can add about how to do it. that hasn't already been done.
Hold down the Power and Home Button for 20 seconds.