For this tutorial we will be using one of the most effective and one of the least known tools called "Low Orbit Ion Cannon", created by Anonymous members from 4chan.org, this program is one of the best for DDoS'ing, and I have successfully used it to DDoS websites. An internet connection as bad as mine (2,500 kb/s) was able to keep a site down for a day with this program running. Remember that this tool will work best with high internet speeds, and try not to go for impossible targets (like Google, Myspace,Yahoo). LOIC is used on a single computer, but with friends it's enough to give sites a great deal of downtime.
Prerequisites: Download LOIC (Low Orbit Ion Cannon).
Step 1: Type the target URL in the URL box.
Step 2: Click lock on.
Step 3: Change the threads to 9001 for maximum efficiency.
Step 4: Click the big button "IMMA FIRIN MAH LAZAR!"
You can download it by searching Google for it